Algebraic notation and substitution
By the end of this chapter you will be able to
- Write algebra using the shorter notation: leave out the times sign, put the number first, and use powers for repeated letters
- Write fractions, brackets and division with letters in the correct form
- Substitute positive numbers into expressions and formulae, using brackets where needed
- Substitute negative numbers into expressions, and keep track of the signs of powers
- Use the words expression, equation, inequality, formula, term, factor and coefficient correctly
You should already know
- Multiplication and division facts, and the order of operations (Chapter 1)
- Using a letter for an unknown number (Year 6, Chapter 4)
- Fractions and decimals (Chapter 2)
Where this leads
- Simplifying and expanding expressions (Chapter 5)
- Solving equations and rearranging formulae (Chapter 6)
National Curriculum for England, Key Stage 3 mathematics, Year 7. Content: A1 notation: ab, 3y, a², a³, a²b, a/b, fractions as coefficients and brackets, A2 substitute numbers into formulae and expressions, including scientific formulae, A3 the vocabulary of expressions, equations, inequalities, terms and factors.
Exercise P Do you remember?
Non-calculatorMethod. Multiplication comes before addition, so work out 3 × 7 first.
- Multiply first. Multiplication is done before addition.3 × 7 = 21
- Then add. Add 4 to the answer.21 + 4 = 25
Check. 25 − 4 = 21, and 21 ÷ 3 = 7. ✓
Watch out. Do not add 7 and 4 first. That gives 33, which is wrong.
Method. Multiply 3 by 4 first, then subtract from 20.
- Multiply. Multiplication is done before subtraction.3 × 4 = 12
- Subtract. Take 12 from 20.20 − 12 = 8
Check. 8 + 12 = 20. ✓
Watch out. Subtracting first gives 17 × 4 = 68, which is wrong.
Method. Work out the brackets first, then multiply.
- Work out the brackets. The brackets are done first.2 + 3 = 5
- Multiply. Multiply 5 by the answer in the brackets.5 × 5 = 25
Check. 25 ÷ 5 = 5. ✓
Watch out. The brackets change the order. Without them, 5 × 2 + 3 = 13.
Method. "Squared" means multiplied by itself.
- Write the multiplication. Four squared is 4 multiplied by itself.4 × 4
- Work out the answer. Four times four is sixteen.4 × 4 = 16
Check. 16 is a square number: 4 × 4 = 16. ✓
Watch out. 4 squared is 16, not 8, which is 4 × 2.
Method. Divide the top number by the bottom number.
- Write the division as a fraction. A fraction line means divide.12/4
- Simplify. Divide the top and bottom by 4.12/4 = 3
Check. 3 × 4 = 12. ✓
Watch out. 12/4 is a whole number here, so it simplifies to 3.
Algebraic notation: multiplying and dividing
In algebra we use letters for numbers we do not yet know. We follow some rules to make the writing short and clear:
- Leave out the × sign: 3 × t is written 3t, and a × b is written ab.
- Put the number first: t × 3 is written 3t.
- Write repeated letters as a power: m × m is m2 (m squared), and n × n × n is n3 (n cubed).
- A division is written as a fraction: a ÷ 3 is a/3.
Key ideas
- Leave out the × sign, and put the number first: 4 × y = 4y.
- Repeated letters are powers: a × a = a2, a × a × a = a3.
- Division with letters is a fraction: m ÷ n = m/n.
- Do not write 3t as t3, because t3 looks like t cubed (t3).
Method. Use the shorter notation rules: leave out the times sign, put the number first, and write repeated letters as powers.
- Leave out the times signs. Algebra does not need the × sign between letters.5 x y
- Put the number first. A number is written in front of the letters.5 is already first
- Write the answer. The letters are written next to each other.5xy
Check. 5 × x × y gives 5xy, with no times signs. ✓
Watch out. Write the number first, then the letters in alphabetical order.
Method. Use the shorter notation rules: leave out the times sign, put the number first, and write repeated letters as powers.
- Group the repeated letters. a appears twice and b appears three times.a × a = a2, b × b × b = b3
- Write them together. Put the powers next to each other.a2 × b3
- Write the answer. The × sign is not written.a2b3
Check. a has power 2 and b has power 3, from the number of times each letter is used. ✓
Watch out. The power belongs to one letter only. a2b3 is not (ab)5.
Method. Use the shorter notation rules: leave out the times sign, put the number first, and write repeated letters as powers.
- Write the division as a fraction. A division by 2 is written with 2 on the bottom.(n + 3)/2
- Check the brackets. The brackets keep n + 3 together, so the whole thing is divided by 2.(n + 3)/2 means (n + 3) ÷ 2
- Write the answer. The fraction line shows the division.(n + 3)/2
Check. Check: with n = 1, (1 + 3) ÷ 2 = 2. ✓
Watch out. Without the brackets, n + 3/2 would mean n + 1.5, which is different.
Watch out
Watch out: 3t is not t3. The number always goes first, so t3 would be read as t cubed. Also, 2 × a × b is written 2ab, not 2a + b.
Exam tip
Grown-up tip. Write a short sum in words first ("3 lots of t") before writing it in letters, so that the pupil sees why the times sign is dropped.
Exercise 4A Writing algebra with multiplication and division
Non-calculatorMethod. Use the shorter notation rules: leave out the times sign, put the number first, and write repeated letters as powers.
- Leave out the times sign. Letters are written next to numbers.3 × a
- Write the number first. The 3 goes in front.3a
Check. 3a means 3 lots of a. ✓
Watch out. Do not write a3.
Method. Use the shorter notation rules: leave out the times sign, put the number first, and write repeated letters as powers.
- Find what is multiplied. Numbers and letters next to each other are multiplied together.y × 7 = 7y
- Put the number first. The number 7 is written before the letter.y × 7 = 7y
Check. 7 lots of y is 7y. ✓
Watch out. Write 7y, not y7.
Method. Use the shorter notation rules: leave out the times sign, put the number first, and write repeated letters as powers.
- Put the number first. The 4 goes at the front.4 b c
- Write the answer. Letters are written next to each other.4bc
Check. 4bc has no times signs. ✓
Watch out. Write the letters in alphabetical order: bc, not cb.
Method. Use the shorter notation rules: leave out the times sign, put the number first, and write repeated letters as powers.
- Find what is multiplied. Numbers and letters next to each other are multiplied together.p × p = p2
- Write repeated letters as a power. p appears twice, so it is squared.p × p = p2
Check. p squared is p × p. ✓
Watch out. p2 is not 2p.
Method. Use the shorter notation rules: leave out the times sign, put the number first, and write repeated letters as powers.
- Find what is multiplied. Numbers and letters next to each other are multiplied together.x × x × x = x3
- Write repeated letters as a power. x appears three times, so it is cubed.x × x × x = x3
Check. x cubed is x × x × x. ✓
Watch out. x3 is not 3x.
Method. Use the shorter notation rules: leave out the times sign, put the number first, and write repeated letters as powers.
- Group the repeated letters. m appears twice.m × m = m2
- Write the answer. Put the number first.2m2n
Check. 2m2n has the number, the squared letter and the other letter. ✓
Watch out. The power 2 belongs only to m, not to the 2 in front.
Method. Use the shorter notation rules: leave out the times sign, put the number first, and write repeated letters as powers.
- Find what is multiplied. Numbers and letters next to each other are multiplied together.a ÷ 3 = a/3
- Write the division as a fraction. Dividing by 3 puts 3 on the bottom.a ÷ 3 = a/3
Check. a/3 is a third of a. ✓
Watch out. Do not write 3/a: that is 3 divided by a.
Method. Use the shorter notation rules: leave out the times sign, put the number first, and write repeated letters as powers.
- Find what is multiplied. Numbers and letters next to each other are multiplied together.5 × (x + 1) = 5(x + 1)
- Keep the brackets. The brackets are kept, and the times sign is left out.5 × (x + 1) = 5(x + 1)
Check. 5(x + 1) means 5 times the whole bracket. ✓
Watch out. Writing 5x + 1 is different: it only multiplies the x.
Method. Use the shorter notation rules: leave out the times sign, put the number first, and write repeated letters as powers.
- Find what is multiplied. Numbers and letters next to each other are multiplied together.4t
- Put the number first. The 4 goes in front.4t
Check. 4t. ✓
Watch out. Do not write t4.
Method. Use the shorter notation rules: leave out the times sign, put the number first, and write repeated letters as powers.
- Find what is multiplied. Numbers and letters next to each other are multiplied together.t × t × 4 = 4t2
- Group the letters and the number. t appears twice.t × t × 4 = 4t2
Check. 4t2. ✓
Watch out. Put the 4 first, then the letter with its power.
Method. Use the shorter notation rules: leave out the times sign, put the number first, and write repeated letters as powers.
- Find what is multiplied. Numbers and letters next to each other are multiplied together.t/4
- Write it as a fraction. Dividing by 4 means the fraction t/4.t/4
Check. t/4. ✓
Watch out. Do not write 4/t.
Method. Use the shorter notation rules: leave out the times sign, put the number first, and write repeated letters as powers.
- Multiply the cost by the number of pens. Total cost = number of pens × cost of each pen.n × p
- Write it in the shorter form. Put the letters in alphabetical order, with no times sign.pn
Check. If n = 2 and p = 35, the cost is 70p, and pn = 70. ✓
Watch out. Write pn, not np, by alphabetical order. Both mean the same.
Method. Use the shorter notation rules: leave out the times sign, put the number first, and write repeated letters as powers.
- Look at each option. A is 2a + b, which adds b. C adds 2. B multiplies everything.A = 2a + b, C = ab + 2, B = 2ab
- Choose the match. Only B multiplies 2, a and b.B
Check. Test with a = 1, b = 3: 2ab = 6, B = 6, A = 5, C = 5. ✓
Watch out. Do not read 2ab as 2a + b. The letters are multiplied.
Method. Use the shorter notation rules: leave out the times sign, put the number first, and write repeated letters as powers.
- Look at the notation. x3 could be read as x cubed, which is a different number.x3 might mean x3
- Write it correctly. Put the number first.3x
Check. If x = 2, then 3x = 6, but x3 = 8. ✓
Watch out. Always put the number first, so that 3x is not confused with x3.
Fractions, brackets and powers in algebra
Algebraic fractions are written with a fraction line. A number on the top of a letter is called the coefficient: in 3y/4, the 3 is multiplied by y, and then the answer is divided by 4.
Brackets keep parts of an expression together. A number outside brackets multiplies the whole bracket: 4(x + 2) means 4 × (x + 2).
Key ideas
- A fraction line is a division: p × 2 ÷ 5 = 2p/5.
- A number outside brackets multiplies everything inside: 4(x + 2) means 4 × (x + 2).
- Coefficient: the number multiplied by a letter, such as 3 in 3y.
- Repeated letters are powers: 2 × a × a × 3 = 6a2.
Method. Use the shorter notation rules: leave out the times sign, put the number first, and write repeated letters as powers.
- Write the multiplication. The 2 is multiplied by p.2 × p = 2p
- Write the division as a fraction. Dividing by 5 puts 5 on the bottom.2p ÷ 5 = 2p/5
- Write the answer. The fraction line shows the division.2p/5
Check. If p = 10, (2 × 10) ÷ 5 = 4, and 2p/5 = 4. ✓
Watch out. Keep the 2 on the top with the p.
Method. Use the shorter notation rules: leave out the times sign, put the number first, and write repeated letters as powers.
- Multiply the length and width. Area = length × width.2a × b
- Write it in the shorter form. Leave out the times sign, and put the number first.2ab
- Write the answer. The area is 2ab.2ab
Check. If a = 3 and b = 2, the area is 2 × 3 × 2 = 12. ✓
Watch out. Do not write 2a + b for an area: that is a sum, not a product.
Method. Use the shorter notation rules: leave out the times sign, put the number first, and write repeated letters as powers.
- Keep the brackets. The bracket stays, and the times sign is left out.4 × (x + 2) = 4(x + 2)
- Check the meaning. The 4 multiplies the whole bracket.4(x + 2) = 4x + 8
- Write the answer. The brackets are kept.4(x + 2)
Check. With x = 1, 4 × (1 + 2) = 12 and 4x + 8 = 12. ✓
Watch out. Writing 4x + 2 would be wrong: the 4 must multiply the whole bracket.
Watch out
Watch out: 4(x + 2) is not 4x + 2. The 4 multiplies both the x and the 2 in the bracket.
Exam tip
Grown-up tip. Use a number to check each expression: put x = 1 into both sides. If the two answers differ, the notation is wrong.
Exercise 4B Fractions, brackets and powers
Non-calculatorMethod. Use the shorter notation rules: leave out the times sign, put the number first, and write repeated letters as powers.
- Group the repeated letters. k appears twice.k × k = k2
- Write the answer. Put the number first.3k2
Check. 3k2 means 3 lots of k × k. ✓
Watch out. The power is on the k only.
Method. Use the shorter notation rules: leave out the times sign, put the number first, and write repeated letters as powers.
- Multiply the numbers. 2 × 2 = 4.4
- Group the letters. a appears three times.a × a × a = a3
Check. 4a3. ✓
Watch out. Multiply the numbers together first, and then write the powers.
Method. Use the shorter notation rules: leave out the times sign, put the number first, and write repeated letters as powers.
- Find what is multiplied. Numbers and letters next to each other are multiplied together.p ÷ 2 = p/2
- Write as a fraction. Half of p is p divided by 2.p ÷ 2 = p/2
Check. p/2 is half of p. ✓
Watch out. Do not write 2/p.
Method. Use the shorter notation rules: leave out the times sign, put the number first, and write repeated letters as powers.
- Write the multiplication. The 3 multiplies y.3y
- Write it as a fraction. Dividing by 4 puts 4 on the bottom.3y/4
Check. 3y/4 is three quarters of y. ✓
Watch out. Write the fraction with the 3 and the y on the top.
Method. Use the shorter notation rules: leave out the times sign, put the number first, and write repeated letters as powers.
- Find what is multiplied. Numbers and letters next to each other are multiplied together.4(x + 1)
- Keep the brackets. The times sign is left out.4(x + 1)
Check. 4(x + 1) with x = 2 is 12. ✓
Watch out. Do not multiply only the x.
Method. Use the shorter notation rules: leave out the times sign, put the number first, and write repeated letters as powers.
- Add the four sides. A square has four sides of length t.t + t + t + t
- Write it in the shorter form. Use 4 times t.4t
Check. 4t: if t = 5 the perimeter is 20. ✓
Watch out. Write 4t, not t4.
Method. Use the shorter notation rules: leave out the times sign, put the number first, and write repeated letters as powers.
- Group the letters. m appears twice.m × m = m2
- Write the answer. The 2 goes first.2m2n
Check. 2m2n. ✓
Watch out. The power belongs to m only.
Method. Use the shorter notation rules: leave out the times sign, put the number first, and write repeated letters as powers.
- Group the letters. q appears twice.q × q = q2
- Write the answer. Put the 5 first.5pq2
Check. 5pq2. ✓
Watch out. Do not write 5p2q.
Method. Use the shorter notation rules: leave out the times sign, put the number first, and write repeated letters as powers.
- Find what is multiplied. Numbers and letters next to each other are multiplied together.a4
- Write as a power. a appears four times.a4
Check. a4. ✓
Watch out. a4 is not 4a.
Method. Use the shorter notation rules: leave out the times sign, put the number first, and write repeated letters as powers.
- Find what is multiplied. Numbers and letters next to each other are multiplied together.b/c
- Write as a fraction. Dividing by c means b over c.b/c
Check. b/c. ✓
Watch out. Do not write c/b.
Method. Use the shorter notation rules: leave out the times sign, put the number first, and write repeated letters as powers.
- Find what is multiplied. Numbers and letters next to each other are multiplied together.2(m − n)
- Keep the brackets. The 2 multiplies the whole bracket.2(m − n)
Check. 2(m − n). ✓
Watch out. Writing 2m − n is wrong.
Method. Use the shorter notation rules: leave out the times sign, put the number first, and write repeated letters as powers.
- Work out the cost of the tickets. 3 tickets cost 3 × c.3c
- Work out the cost of the drinks. 2 drinks cost 2 × d.2d
- Add them. The total is the sum of the two costs.3c + 2d
Check. If c = 6 and d = 3, the cost is 18 + 6 = 24, and 3c + 2d = 24. ✓
Watch out. Do not write 3c + 2d as 5cd.
Method. Use the shorter notation rules: leave out the times sign, put the number first, and write repeated letters as powers.
- Use a = 3 in the bracket form. Work out the bracket first.2(3 + 1) = 2 × 4 = 8
- Use a = 3 in the other form. Multiply first, then add.2 × 3 + 1 = 7
- Compare. The two answers are different, so the expressions are not the same.No. 2(3 + 1) = 8 but 2 × 3 + 1 = 7
Check. The 2 must multiply the 1 inside the bracket, which 2a + 1 does not do. ✓
Watch out. The 2 multiplies the whole bracket: 2(a + 1) = 2a + 2.
Method. Use the shorter notation rules: leave out the times sign, put the number first, and write repeated letters as powers.
- Leave out the times signs. Letters next to each other mean multiplication.4 × x × y = 4xy
- Check. Both forms mean the same product.4xy
Check. With x = 2 and y = 3, both are 24. ✓
Watch out. The times sign is left out only in algebra, where letters show multiplication.
Substituting positive numbers
To substitute, replace each letter with its value. Use brackets around each value, so that the working is clear: 3x when x = 4 becomes 3 × (4) = 12.
A formula is an equation that links quantities. In science, the formula v = u + at links the final speed v, the starting speed u, the acceleration a and the time t.
Key ideas
- Substitute: replace each letter with its value, using brackets.
- Follow BIDMAS: powers first, then × and ÷, then + and −.
- A formula can be used for many different values of the letters.
- Check your answer by working out the other way, or by estimating.
Method. Replace each letter with its value, using brackets. Then work out the answer, following the order of operations.
- Write the expression or formula. This is what we need to work out.3x + 2
- Replace each letter with its value. Brackets keep a negative number together, so its sign is not lost.3 × 4 + 2
- Work out the calculation, in order. Powers first, then multiplication and division, then addition and subtraction.3 × 4 = 12; 12 + 2 = 14
- Write the answer. Check that it is the value of the expression for the values given.14
Check. Check by replacing the letters again and working out the sum a second time. ✓
Watch out. Put brackets around each value before you multiply or square. Without brackets, −3 squared looks like −(3 squared).
Method. Replace each letter with its value, using brackets. Then work out the answer, following the order of operations.
- Write the expression or formula. This is what we need to work out.5ab
- Replace each letter with its value. Brackets keep a negative number together, so its sign is not lost.5 × 2 × 3
- Work out the calculation, in order. Powers first, then multiplication and division, then addition and subtraction.5 × 2 = 10; 10 × 3 = 30
- Write the answer. Check that it is the value of the expression for the values given.30
Check. Check by replacing the letters again and working out the sum a second time. ✓
Watch out. Put brackets around each value before you multiply or square. Without brackets, −3 squared looks like −(3 squared).
Method. Replace each letter with its value, using brackets. Then work out the answer, following the order of operations.
- Write the expression or formula. This is what we need to work out.v = u + at
- Replace each letter with its value. Brackets keep a negative number together, so its sign is not lost.3 + 4 × 5
- Work out the calculation, in order. Powers first, then multiplication and division, then addition and subtraction.4 × 5 = 20; 3 + 20 = 23
- Write the answer. Check that it is the value of the expression for the values given.23
Check. Check by replacing the letters again and working out the sum a second time. ✓
Watch out. Put brackets around each value before you multiply or square. Without brackets, −3 squared looks like −(3 squared).
Watch out
Watch out: 2 × 3 + 1 is 7, not 9. Multiplication comes before addition, so work out 2 × 3 first.
Exam tip
Grown-up tip. Ask the pupil to write the value in brackets above each letter before they start, so that they never forget a number.
Exercise 4C Substituting positive numbers
Non-calculatorMethod. Replace each letter with its value, using brackets. Then work out the answer, following the order of operations.
- Write the expression or formula. This is what we need to work out.2x + 5
- Replace each letter with its value. Brackets keep a negative number together, so its sign is not lost.2 × 3 + 5
- Work out the calculation, in order. Powers first, then multiplication and division, then addition and subtraction.2 × 3 = 6; 6 + 5 = 11
- Write the answer. Check that it is the value of the expression for the values given.11
Check. Check by replacing the letters again and working out the sum a second time. ✓
Watch out. Put brackets around each value before you multiply or square. Without brackets, −3 squared looks like −(3 squared).
Method. Replace each letter with its value, using brackets. Then work out the answer, following the order of operations.
- Write the expression or formula. This is what we need to work out.4a − 7
- Replace each letter with its value. Brackets keep a negative number together, so its sign is not lost.4 × 6 − 7
- Work out the calculation, in order. Powers first, then multiplication and division, then addition and subtraction.4 × 6 = 24; 24 − 7 = 17
- Write the answer. Check that it is the value of the expression for the values given.17
Check. Check by replacing the letters again and working out the sum a second time. ✓
Watch out. Put brackets around each value before you multiply or square. Without brackets, −3 squared looks like −(3 squared).
Method. Replace each letter with its value, using brackets. Then work out the answer, following the order of operations.
- Write the expression or formula. This is what we need to work out.3y2
- Replace each letter with its value. Brackets keep a negative number together, so its sign is not lost.3 × 22
- Work out the calculation, in order. Powers first, then multiplication and division, then addition and subtraction.22 = 4; 3 × 4 = 12
- Write the answer. Check that it is the value of the expression for the values given.12
Check. Check by replacing the letters again and working out the sum a second time. ✓
Watch out. Put brackets around each value before you multiply or square. Without brackets, −3 squared looks like −(3 squared).
Method. Replace each letter with its value, using brackets. Then work out the answer, following the order of operations.
- Write the expression or formula. This is what we need to work out.5(p + 3)
- Replace each letter with its value. Brackets keep a negative number together, so its sign is not lost.5 × (1 + 3)
- Work out the calculation, in order. Powers first, then multiplication and division, then addition and subtraction.1 + 3 = 4; 5 × 4 = 20
- Write the answer. Check that it is the value of the expression for the values given.20
Check. Check by replacing the letters again and working out the sum a second time. ✓
Watch out. Put brackets around each value before you multiply or square. Without brackets, −3 squared looks like −(3 squared).
Method. Replace each letter with its value, using brackets. Then work out the answer, following the order of operations.
- Write the expression or formula. This is what we need to work out.2ab
- Replace each letter with its value. Brackets keep a negative number together, so its sign is not lost.2 × 4 × 5
- Work out the calculation, in order. Powers first, then multiplication and division, then addition and subtraction.2 × 4 = 8; 8 × 5 = 40
- Write the answer. Check that it is the value of the expression for the values given.40
Check. Check by replacing the letters again and working out the sum a second time. ✓
Watch out. Put brackets around each value before you multiply or square. Without brackets, −3 squared looks like −(3 squared).
Method. Replace each letter with its value, using brackets. Then work out the answer, following the order of operations.
- Write the expression or formula. This is what we need to work out.x/4 + 1
- Replace each letter with its value. Brackets keep a negative number together, so its sign is not lost.12 ÷ 4 + 1
- Work out the calculation, in order. Powers first, then multiplication and division, then addition and subtraction.12 ÷ 4 = 3; 3 + 1 = 4
- Write the answer. Check that it is the value of the expression for the values given.4
Check. Check by replacing the letters again and working out the sum a second time. ✓
Watch out. Put brackets around each value before you multiply or square. Without brackets, −3 squared looks like −(3 squared).
Method. Replace each letter with its value, using brackets. Then work out the answer, following the order of operations.
- Write the expression or formula. This is what we need to work out.P = 2(l + w)
- Replace each letter with its value. Brackets keep a negative number together, so its sign is not lost.2 × (7 + 3)
- Work out the calculation, in order. Powers first, then multiplication and division, then addition and subtraction.7 + 3 = 10; 2 × 10 = 20
- Write the answer. Check that it is the value of the expression for the values given.20
Check. Check by replacing the letters again and working out the sum a second time. ✓
Watch out. Put brackets around each value before you multiply or square. Without brackets, −3 squared looks like −(3 squared).
Method. Replace each letter with its value, using brackets. Then work out the answer, following the order of operations.
- Write the expression or formula. This is what we need to work out.s = d/t
- Replace each letter with its value. Brackets keep a negative number together, so its sign is not lost.150 ÷ 2.5
- Work out the calculation, in order. Powers first, then multiplication and division, then addition and subtraction.150 ÷ 2.5 = 60
- Write the answer. Check that it is the value of the expression for the values given.60
Check. Check by replacing the letters again and working out the sum a second time. ✓
Watch out. Put brackets around each value before you multiply or square. Without brackets, −3 squared looks like −(3 squared).
Method. Replace each letter with its value, using brackets. Then work out the answer, following the order of operations.
- Write the expression or formula. This is what we need to work out.F = 9C/5 + 32
- Replace each letter with its value. Brackets keep a negative number together, so its sign is not lost.9 × 20 ÷ 5 + 32
- Work out the calculation, in order. Powers first, then multiplication and division, then addition and subtraction.9 × 20 = 180; 180 ÷ 5 = 36; 36 + 32 = 68
- Write the answer. Check that it is the value of the expression for the values given.68
Check. Check by replacing the letters again and working out the sum a second time. ✓
Watch out. Put brackets around each value before you multiply or square. Without brackets, −3 squared looks like −(3 squared).
Method. Replace each letter with its value, using brackets. Then work out the answer, following the order of operations.
- Write the expression or formula. This is what we need to work out.v = u + at
- Replace each letter with its value. Brackets keep a negative number together, so its sign is not lost.12 + 2 × 6
- Work out the calculation, in order. Powers first, then multiplication and division, then addition and subtraction.2 × 6 = 12; 12 + 12 = 24
- Write the answer. Check that it is the value of the expression for the values given.24
Check. Check by replacing the letters again and working out the sum a second time. ✓
Watch out. Put brackets around each value before you multiply or square. Without brackets, −3 squared looks like −(3 squared).
Method. Replace each letter with its value, using brackets. Then work out the answer, following the order of operations.
- Write the expression or formula. This is what we need to work out.x2 + 2x
- Replace each letter with its value. Brackets keep a negative number together, so its sign is not lost.52 + 2 × 5
- Work out the calculation, in order. Powers first, then multiplication and division, then addition and subtraction.52 = 25; 2 × 5 = 10; 25 + 10 = 35
- Write the answer. Check that it is the value of the expression for the values given.35
Check. Check by replacing the letters again and working out the sum a second time. ✓
Watch out. Put brackets around each value before you multiply or square. Without brackets, −3 squared looks like −(3 squared).
Method. Replace each letter with its value, using brackets. Then work out the answer, following the order of operations.
- Write the expression or formula. This is what we need to work out.3(x + y)
- Replace each letter with its value. Brackets keep a negative number together, so its sign is not lost.3 × (2 + 7)
- Work out the calculation, in order. Powers first, then multiplication and division, then addition and subtraction.2 + 7 = 9; 3 × 9 = 27
- Write the answer. Check that it is the value of the expression for the values given.27
Check. Check by replacing the letters again and working out the sum a second time. ✓
Watch out. Put brackets around each value before you multiply or square. Without brackets, −3 squared looks like −(3 squared).
Method. Only the n is squared. The 2 is not squared.
- Square the n. The power 2 belongs to n only.32 = 9
- Multiply by 2. The 2 in front multiplies the answer.2 × 9 = 18
- Compare. Sam squared 2n, which is (2n)2 = 36, a different answer.18, so Sam is wrong
Check. With n = 3: 2 × 32 = 18. ✓
Watch out. Look at the brackets. 2n2 and (2n)2 are different expressions.
Substituting negative numbers and powers
Put every negative value in brackets when you substitute, so that you can see its sign. For example, 3x when x = −2 is 3 × (−2) = −6.
Be careful with powers. (−4)2 means −4 × −4 = 16, but −42 means −(4 × 4) = −16.
Key ideas
- Put a negative value in brackets: x = −2 becomes (−2).
- A negative times a negative is positive: (−2) × (−2) = 4.
- Only the number directly before a power is squared or cubed: −x2 means −(x × x).
- Subtracting a negative is the same as adding: a − (−6) = a + 6.
Method. Replace each letter with its value, using brackets. Then work out the answer, following the order of operations.
- Write the expression or formula. This is what we need to work out.3x − 5
- Replace each letter with its value. Brackets keep a negative number together, so its sign is not lost.3 × (−2) − 5
- Work out the calculation, in order. Powers first, then multiplication and division, then addition and subtraction.3 × (−2) = −6; −6 − 5 = −11
- Write the answer. Check that it is the value of the expression for the values given.−11
Check. Check by replacing the letters again and working out the sum a second time. ✓
Watch out. Put brackets around each value before you multiply or square. Without brackets, −3 squared looks like −(3 squared).
Method. Replace each letter with its value, using brackets. Then work out the answer, following the order of operations.
- Write the expression or formula. This is what we need to work out.x2
- Replace each letter with its value. Brackets keep a negative number together, so its sign is not lost.(−4)2
- Work out the calculation, in order. Powers first, then multiplication and division, then addition and subtraction.(−4) × (−4) = 16
- Write the answer. Check that it is the value of the expression for the values given.16
Check. Check by replacing the letters again and working out the sum a second time. ✓
Watch out. Put brackets around each value before you multiply or square. Without brackets, −3 squared looks like −(3 squared).
Method. Replace each letter with its value, using brackets. Then work out the answer, following the order of operations.
- Write the expression or formula. This is what we need to work out.2a − b
- Replace each letter with its value. Brackets keep a negative number together, so its sign is not lost.2 × (−3) − (−6)
- Work out the calculation, in order. Powers first, then multiplication and division, then addition and subtraction.2 × (−3) = −6; −6 − (−6) = −6 + 6 = 0
- Write the answer. Check that it is the value of the expression for the values given.0
Check. Check by replacing the letters again and working out the sum a second time. ✓
Watch out. Put brackets around each value before you multiply or square. Without brackets, −3 squared looks like −(3 squared).
Watch out
Watch out: −x2 is −(x × x), so if x = 5 then −x2 = −25, not 25. Also (−3)2 = 9, but −32 = −9.
Exam tip
Grown-up tip. Ask the pupil to write the value in brackets before they do anything else, and to say "a negative times a negative is positive" out loud.
Exercise 4D Substituting negative numbers
Non-calculatorMethod. Replace each letter with its value, using brackets. Then work out the answer, following the order of operations.
- Write the expression or formula. This is what we need to work out.4x + 1
- Replace each letter with its value. Brackets keep a negative number together, so its sign is not lost.4 × (−3) + 1
- Work out the calculation, in order. Powers first, then multiplication and division, then addition and subtraction.4 × (−3) = −12; −12 + 1 = −11
- Write the answer. Check that it is the value of the expression for the values given.−11
Check. Check by replacing the letters again and working out the sum a second time. ✓
Watch out. Put brackets around each value before you multiply or square. Without brackets, −3 squared looks like −(3 squared).
Method. Replace each letter with its value, using brackets. Then work out the answer, following the order of operations.
- Write the expression or formula. This is what we need to work out.2a − 7
- Replace each letter with its value. Brackets keep a negative number together, so its sign is not lost.2 × (−5) − 7
- Work out the calculation, in order. Powers first, then multiplication and division, then addition and subtraction.2 × (−5) = −10; −10 − 7 = −17
- Write the answer. Check that it is the value of the expression for the values given.−17
Check. Check by replacing the letters again and working out the sum a second time. ✓
Watch out. Put brackets around each value before you multiply or square. Without brackets, −3 squared looks like −(3 squared).
Method. Replace each letter with its value, using brackets. Then work out the answer, following the order of operations.
- Write the expression or formula. This is what we need to work out.x2
- Replace each letter with its value. Brackets keep a negative number together, so its sign is not lost.(−6)2
- Work out the calculation, in order. Powers first, then multiplication and division, then addition and subtraction.(−6) × (−6) = 36
- Write the answer. Check that it is the value of the expression for the values given.36
Check. Check by replacing the letters again and working out the sum a second time. ✓
Watch out. Put brackets around each value before you multiply or square. Without brackets, −3 squared looks like −(3 squared).
Method. Replace each letter with its value, using brackets. Then work out the answer, following the order of operations.
- Write the expression or formula. This is what we need to work out.−x2
- Replace each letter with its value. Brackets keep a negative number together, so its sign is not lost.−(52)
- Work out the calculation, in order. Powers first, then multiplication and division, then addition and subtraction.52 = 25; −25
- Write the answer. Check that it is the value of the expression for the values given.−25
Check. Check by replacing the letters again and working out the sum a second time. ✓
Watch out. Put brackets around each value before you multiply or square. Without brackets, −3 squared looks like −(3 squared).
Method. Replace each letter with its value, using brackets. Then work out the answer, following the order of operations.
- Write the expression or formula. This is what we need to work out.3y − y2
- Replace each letter with its value. Brackets keep a negative number together, so its sign is not lost.3 × (−2) − (−2)2
- Work out the calculation, in order. Powers first, then multiplication and division, then addition and subtraction.3 × (−2) = −6; (−2)2 = 4; −6 − 4 = −10
- Write the answer. Check that it is the value of the expression for the values given.−10
Check. Check by replacing the letters again and working out the sum a second time. ✓
Watch out. Put brackets around each value before you multiply or square. Without brackets, −3 squared looks like −(3 squared).
Method. Replace each letter with its value, using brackets. Then work out the answer, following the order of operations.
- Write the expression or formula. This is what we need to work out.a − b
- Replace each letter with its value. Brackets keep a negative number together, so its sign is not lost.(−1) − (−6)
- Work out the calculation, in order. Powers first, then multiplication and division, then addition and subtraction.(−1) − (−6) = −1 + 6 = 5
- Write the answer. Check that it is the value of the expression for the values given.5
Check. Check by replacing the letters again and working out the sum a second time. ✓
Watch out. Put brackets around each value before you multiply or square. Without brackets, −3 squared looks like −(3 squared).
Method. Replace each letter with its value, using brackets. Then work out the answer, following the order of operations.
- Write the expression or formula. This is what we need to work out.5(x − 4)
- Replace each letter with its value. Brackets keep a negative number together, so its sign is not lost.5 × ((−2) − 4)
- Work out the calculation, in order. Powers first, then multiplication and division, then addition and subtraction.(−2) − 4 = −6; 5 × (−6) = −30
- Write the answer. Check that it is the value of the expression for the values given.−30
Check. Check by replacing the letters again and working out the sum a second time. ✓
Watch out. Put brackets around each value before you multiply or square. Without brackets, −3 squared looks like −(3 squared).
Method. Replace each letter with its value, using brackets. Then work out the answer, following the order of operations.
- Write the expression or formula. This is what we need to work out.mn
- Replace each letter with its value. Brackets keep a negative number together, so its sign is not lost.(−4) × (−7)
- Work out the calculation, in order. Powers first, then multiplication and division, then addition and subtraction.(−4) × (−7) = 28
- Write the answer. Check that it is the value of the expression for the values given.28
Check. Check by replacing the letters again and working out the sum a second time. ✓
Watch out. Put brackets around each value before you multiply or square. Without brackets, −3 squared looks like −(3 squared).
Method. Replace each letter with its value, using brackets. Then work out the answer, following the order of operations.
- Write the expression or formula. This is what we need to work out.t/x
- Replace each letter with its value. Brackets keep a negative number together, so its sign is not lost.12 ÷ (−3)
- Work out the calculation, in order. Powers first, then multiplication and division, then addition and subtraction.12 ÷ (−3) = −4
- Write the answer. Check that it is the value of the expression for the values given.−4
Check. Check by replacing the letters again and working out the sum a second time. ✓
Watch out. Put brackets around each value before you multiply or square. Without brackets, −3 squared looks like −(3 squared).
Method. Replace each letter with its value, using brackets. Then work out the answer, following the order of operations.
- Write the expression or formula. This is what we need to work out.p3
- Replace each letter with its value. Brackets keep a negative number together, so its sign is not lost.(−2)3
- Work out the calculation, in order. Powers first, then multiplication and division, then addition and subtraction.(−2) × (−2) × (−2) = −8
- Write the answer. Check that it is the value of the expression for the values given.−8
Check. Check by replacing the letters again and working out the sum a second time. ✓
Watch out. Put brackets around each value before you multiply or square. Without brackets, −3 squared looks like −(3 squared).
Method. Replace each letter with its value, using brackets. Then work out the answer, following the order of operations.
- Write the expression or formula. This is what we need to work out.(−p)3
- Replace each letter with its value. Brackets keep a negative number together, so its sign is not lost.(−(−2))3
- Work out the calculation, in order. Powers first, then multiplication and division, then addition and subtraction.−(−2) = 2; 23 = 8
- Write the answer. Check that it is the value of the expression for the values given.8
Check. Check by replacing the letters again and working out the sum a second time. ✓
Watch out. Put brackets around each value before you multiply or square. Without brackets, −3 squared looks like −(3 squared).
Method. Replace each letter with its value, using brackets. Then work out the answer, following the order of operations.
- Write the expression or formula. This is what we need to work out.2a + 3b
- Replace each letter with its value. Brackets keep a negative number together, so its sign is not lost.2 × (−4) + 3 × 5
- Work out the calculation, in order. Powers first, then multiplication and division, then addition and subtraction.2 × (−4) = −8; 3 × 5 = 15; −8 + 15 = 7
- Write the answer. Check that it is the value of the expression for the values given.7
Check. Check by replacing the letters again and working out the sum a second time. ✓
Watch out. Put brackets around each value before you multiply or square. Without brackets, −3 squared looks like −(3 squared).
Method. Put the negative number in brackets, then square it.
- Write the power with brackets. x = −3, so x2 = (−3)2.(−3)2
- Multiply two negatives. A negative times a negative is positive.(−3) × (−3) = 9
- Compare. Sam forgot that the square is applied to the whole number.x2 = 9, not −9
Check. (−3)2 = 9, which is positive. ✓
Watch out. A squared number is never negative, whatever the sign of x.
The words of algebra
An expression has no equals sign, such as 3x + 4. An equation has an equals sign, such as 3x + 4 = 10. An inequality uses a sign such as <, > ≤ or ≥, such as 3x + 4 > 10. A formula is an equation that links two or more quantities, such as A = lw.
A term is a part of an expression separated by + or −: in 3ab + 4c − 5 there are three terms. A factor multiplies to make a term: the factors of 3ab are 3, a and b. The coefficient is the number in front of a letter: in 7x − 2y, the coefficient of x is 7.
Key ideas
- Expression: no equals sign (3x + 4). Equation: has = (3x + 4 = 10). Inequality: has <, >, ≤ or ≥.
- Formula: an equation that links quantities, such as A = lw.
- Term: a part separated by + or −. The terms of 3ab + 4c − 5 are 3ab, 4c and −5.
- Factor: a number or letter multiplied to make a term. Coefficient: the number in front of a letter.
Method. Use the shorter notation rules: leave out the times sign, put the number first, and write repeated letters as powers.
- Look for an equals sign. An equation has =.2x + 3 has no =; 2x + 3 = 11 has =; 2x + 3 > 11 has >
- Check the others. The > sign makes an inequality, and no sign makes an expression.2x + 3 is an expression; 2x + 3 > 11 is an inequality
- Write the answer. Only one has an equals sign.2x + 3 = 11
Check. 2x + 3 = 11 has an equals sign. ✓
Watch out. An equation has an equals sign. An inequality uses < > ≤ or ≥.
Method. Use the shorter notation rules: leave out the times sign, put the number first, and write repeated letters as powers.
- Find the parts separated by + or −. Each term is a part of the expression between the + and − signs.3ab, 4c and −5
- Count them. There are three terms.3 terms
- Write the answer. Keep the sign with the term.3ab, 4c and −5
Check. Three terms: 3ab, 4c and −5. ✓
Watch out. The − sign belongs with the 5, so the third term is −5, not 5.
Method. Use the shorter notation rules: leave out the times sign, put the number first, and write repeated letters as powers.
- Find the numbers and letters multiplied together. A factor multiplies to make the term.4 × m × n = 4mn
- List the factors. The factors are the numbers and letters that are multiplied.4, m and n
- Write the answer. Each factor is a separate multiplication.4, m and n
Check. 4 × m × n = 4mn. ✓
Watch out. A factor is one of the things multiplied, not a sum.
Watch out
Watch out: x + 2 = 7 is an equation, but x + 2 is only an expression. Do not call every line of algebra an equation.
Exam tip
Grown-up tip. Ask the pupil to point at the = or < sign in each line. The sign tells them which word to use.
Exercise 4E Expressions, equations, terms and factors
Non-calculatorMethod. An expression has no equals sign and no inequality sign.
- Look for a sign. There is no = and no <, >, ≤ or ≥.4x − 1: no sign that compares
- Name it. So it is an expression.expression
Check. Expressions just hold a value. ✓
Watch out. It does not state a fact, so it is not an equation.
Method. An equation has an equals sign.
- Look for =. There is an = sign.4x − 1 = 7
- Name it. So it is an equation.equation
Check. It is true when x = 2. ✓
Watch out. An equation has exactly one = sign here.
Method. An inequality uses a sign such as <.
- Look for <. The sign is <, which compares two values.4x − 1 < 7
- Name it. So it is an inequality.inequality
Check. x = 1 makes it true, x = 5 makes it false. ✓
Watch out. An inequality is not an equation, even though it has a sign between two expressions.
Method. A formula is an equation that links quantities.
- Look at what it links. A is the area, and l and w are the length and width.A = lw
- Name it. So it is a formula.formula
Check. A rectangle with l = 3 and w = 2 has area 6. ✓
Watch out. A formula links quantities and has a letter for each quantity.
Method. The terms are the parts between the + and − signs.
- Find the parts. Split the expression at each + or −.5a, 3b, −2
- Write them. Keep the sign with each term.5a, 3b and −2
Check. Three terms. ✓
Watch out. The − belongs to the 2.
Method. The coefficient is the number in front of the letter.
- Find the letter. The letter is y.6y
- Read the number in front. The number in front of y is 6.6
Check. 6 lots of y. ✓
Watch out. The −4 is a number on its own, not a coefficient of y.
Method. −x means −1 × x, so the coefficient is −1.
- Write the term with its 1. A letter on its own has coefficient 1.−x = −1x
- Read the number in front. The number in front of x is −1.−1
Check. −1 × x = −x. ✓
Watch out. Do not say 0 or 1 here. The minus sign belongs to the coefficient.
Method. A factor is one of the numbers or letters that are multiplied to make the term.
- Write the multiplication. The term is made by multiplying.4 × m × n
- List them. Each number or letter is a factor.4, m and n
Check. 4 × m × n = 4mn. ✓
Watch out. A factor is multiplied, not added.
Method. Look at which one has no equals sign and no inequality sign.
- Look at the signs. Three have =, or <.p + q = 7, 3p = 9, 2x < 10
- Find the odd one. a + b has no equals or inequality sign, so it is an expression.a + b
Check. It is the only expression: the other three are equations or an inequality. ✓
Watch out. Look for the sign that compares two values.
Method. Write the operations in order.
- Multiply n by 3. The number n is multiplied by 3.3n
- Add 5. Add 5 to the answer.3n + 5
Check. If n = 2, the answer is 3 × 2 + 5 = 11. ✓
Watch out. Write 3n, not n3. Keep the + 5 at the end.
Method. Count the parts separated by + or − signs.
- Find the terms. Each part between the + and − signs is a term.4a, 2b, −7c, 5
- Count them. There are four terms.4
Check. 4a, 2b, −7c and 5: four terms. ✓
Watch out. The 5 is a term too, even though it has no letter.
Method. Think about what x3 could mean.
- Look at x3. x3 could be read as x cubed, which is x3.x3 might mean x3
- Write 3x. The number is written first, so the meaning is clear.3x
Check. With x = 2, 3x = 6, but x3 = 8. ✓
Watch out. Always put the number first in algebra.
Chapter summary
| Skill | Method |
|---|---|
| Shorter notation | Leave out the times sign and put the number first: 4 × y = 4y |
| Powers in algebra | Repeated letters: a × a = a2, a × a × a = a3 |
| Division | Write a division with letters as a fraction: m ÷ n = m/n |
| Brackets | A number outside a bracket multiplies everything inside: 4(x + 2) |
| Substitution | Replace each letter with its value in brackets, then use BIDMAS |
| Negative values | Put negative values in brackets; a negative times a negative is positive |
| Expression, equation, inequality | No sign; has =; has < > ≤ or ≥ |
| Terms, factors, coefficients | Terms are separated by + or −; factors multiply to make a term; the coefficient is the number in front of a letter |
Can you…?
- I can write algebra with the times sign left out, and the number first
- I can write repeated letters as powers, and divisions as fractions
- I can substitute positive numbers into expressions and formulae
- I can substitute negative numbers, using brackets, and handle powers of negative numbers
- I can use the words expression, equation, inequality and formula correctly
- I can name the terms, factors and coefficients of an expression
Key terms
- expression
- a collection of terms with no equals sign, such as 3x + 4
- equation
- a statement with an equals sign, such as 3x + 4 = 10
- inequality
- a statement using <, >, ≤ or ≥, such as 3x + 4 > 10
- formula
- an equation that links quantities, such as A = lw
- substitute
- replace a letter with a number
- term
- a part of an expression separated by + or − signs
- factor
- a number or letter that is multiplied to make a term
- coefficient
- the number in front of a letter, such as 7 in 7x
Exercise 4Z Check-up
Non-calculatorMethod. Replace each letter with its value, using brackets. Then work out the answer, following the order of operations.
- Write the expression or formula. This is what we need to work out.3x + 7
- Replace each letter with its value. Brackets keep a negative number together, so its sign is not lost.3 × 4 + 7
- Work out the calculation, in order. Powers first, then multiplication and division, then addition and subtraction.3 × 4 = 12; 12 + 7 = 19
- Write the answer. Check that it is the value of the expression for the values given.19
Check. Check by replacing the letters again and working out the sum a second time. ✓
Watch out. Put brackets around each value before you multiply or square. Without brackets, −3 squared looks like −(3 squared).
Method. Replace each letter with its value, using brackets. Then work out the answer, following the order of operations.
- Write the expression or formula. This is what we need to work out.5 − 2y
- Replace each letter with its value. Brackets keep a negative number together, so its sign is not lost.5 − 2 × (−3)
- Work out the calculation, in order. Powers first, then multiplication and division, then addition and subtraction.2 × (−3) = −6; 5 − (−6) = 11
- Write the answer. Check that it is the value of the expression for the values given.11
Check. Check by replacing the letters again and working out the sum a second time. ✓
Watch out. Put brackets around each value before you multiply or square. Without brackets, −3 squared looks like −(3 squared).
Method. Replace each letter with its value, using brackets. Then work out the answer, following the order of operations.
- Write the expression or formula. This is what we need to work out.a2 + b
- Replace each letter with its value. Brackets keep a negative number together, so its sign is not lost.(−5)2 + 3
- Work out the calculation, in order. Powers first, then multiplication and division, then addition and subtraction.(−5)2 = 25; 25 + 3 = 28
- Write the answer. Check that it is the value of the expression for the values given.28
Check. Check by replacing the letters again and working out the sum a second time. ✓
Watch out. Put brackets around each value before you multiply or square. Without brackets, −3 squared looks like −(3 squared).
Method. Replace each letter with its value, using brackets. Then work out the answer, following the order of operations.
- Write the expression or formula. This is what we need to work out.4(m − 2n)
- Replace each letter with its value. Brackets keep a negative number together, so its sign is not lost.4 × (7 − 2 × (−1))
- Work out the calculation, in order. Powers first, then multiplication and division, then addition and subtraction.2 × (−1) = −2; 7 − (−2) = 9; 4 × 9 = 36
- Write the answer. Check that it is the value of the expression for the values given.36
Check. Check by replacing the letters again and working out the sum a second time. ✓
Watch out. Put brackets around each value before you multiply or square. Without brackets, −3 squared looks like −(3 squared).
Method. Replace each letter with its value, using brackets. Then work out the answer, following the order of operations.
- Write the expression or formula. This is what we need to work out.xy/2
- Replace each letter with its value. Brackets keep a negative number together, so its sign is not lost.6 × 5 ÷ 2
- Work out the calculation, in order. Powers first, then multiplication and division, then addition and subtraction.6 × 5 = 30; 30 ÷ 2 = 15
- Write the answer. Check that it is the value of the expression for the values given.15
Check. Check by replacing the letters again and working out the sum a second time. ✓
Watch out. Put brackets around each value before you multiply or square. Without brackets, −3 squared looks like −(3 squared).
Method. Replace each letter with its value, using brackets. Then work out the answer, following the order of operations.
- Write the expression or formula. This is what we need to work out.ab − c
- Replace each letter with its value. Brackets keep a negative number together, so its sign is not lost.1.5 × 4 − 2
- Work out the calculation, in order. Powers first, then multiplication and division, then addition and subtraction.1.5 × 4 = 6; 6 − 2 = 4
- Write the answer. Check that it is the value of the expression for the values given.4
Check. Check by replacing the letters again and working out the sum a second time. ✓
Watch out. Put brackets around each value before you multiply or square. Without brackets, −3 squared looks like −(3 squared).
Method. Replace each letter with its value, using brackets. Then work out the answer, following the order of operations.
- Write the expression or formula. This is what we need to work out.t2 − t
- Replace each letter with its value. Brackets keep a negative number together, so its sign is not lost.(−3)2 − (−3)
- Work out the calculation, in order. Powers first, then multiplication and division, then addition and subtraction.(−3)2 = 9; 9 − (−3) = 12
- Write the answer. Check that it is the value of the expression for the values given.12
Check. Check by replacing the letters again and working out the sum a second time. ✓
Watch out. Put brackets around each value before you multiply or square. Without brackets, −3 squared looks like −(3 squared).
Method. Use the shorter notation rules: leave out the times sign, put the number first, and write repeated letters as powers.
- Group the repeated letters. p appears twice.p × p = p2
- Write the answer. Put the number first.3p2q
Check. 3p2q. ✓
Watch out. The power is on p only.
Method. Use the shorter notation rules: leave out the times sign, put the number first, and write repeated letters as powers.
- Find what is multiplied. Numbers and letters next to each other are multiplied together.(x + 2)/3
- Write the division as a fraction. Dividing by 3 puts 3 on the bottom, and the brackets stay.(x + 2)/3
Check. (x + 2)/3 with x = 1 is 1. ✓
Watch out. Keep the brackets: x + 2/3 is different.
Method. Replace each letter with its value, using brackets. Then work out the answer, following the order of operations.
- Write the expression or formula. This is what we need to work out.2x3
- Replace each letter with its value. Brackets keep a negative number together, so its sign is not lost.2 × 23
- Work out the calculation, in order. Powers first, then multiplication and division, then addition and subtraction.23 = 8; 2 × 8 = 16
- Write the answer. Check that it is the value of the expression for the values given.16
Check. Check by replacing the letters again and working out the sum a second time. ✓
Watch out. Put brackets around each value before you multiply or square. Without brackets, −3 squared looks like −(3 squared).
Total 19 marks. Allow about 19 minutes.
Exercise 4X Practice test questions
Non-calculatorMethod. Use the shorter notation rules: leave out the times sign, put the number first, and write repeated letters as powers.
- Find what is multiplied. Numbers and letters next to each other are multiplied together.6mn
- Leave out the times signs. Letters are written next to each other.6mn
Check. 6mn. ✓
Watch out. Put the number first.
Method. Replace each letter with its value, using brackets. Then work out the answer, following the order of operations.
- Write the expression or formula. This is what we need to work out.4x − 3
- Replace each letter with its value. Brackets keep a negative number together, so its sign is not lost.4 × 5 − 3
- Work out the calculation, in order. Powers first, then multiplication and division, then addition and subtraction.4 × 5 = 20; 20 − 3 = 17
- Write the answer. Check that it is the value of the expression for the values given.17
Check. Check by replacing the letters again and working out the sum a second time. ✓
Watch out. Put brackets around each value before you multiply or square. Without brackets, −3 squared looks like −(3 squared).
Method. Replace each letter with its value, using brackets. Then work out the answer, following the order of operations.
- Write the expression or formula. This is what we need to work out.2a + 5b
- Replace each letter with its value. Brackets keep a negative number together, so its sign is not lost.2 × 3 + 5 × 4
- Work out the calculation, in order. Powers first, then multiplication and division, then addition and subtraction.2 × 3 = 6; 5 × 4 = 20; 6 + 20 = 26
- Write the answer. Check that it is the value of the expression for the values given.26
Check. Check by replacing the letters again and working out the sum a second time. ✓
Watch out. Put brackets around each value before you multiply or square. Without brackets, −3 squared looks like −(3 squared).
Method. Replace each letter with its value, using brackets. Then work out the answer, following the order of operations.
- Write the expression or formula. This is what we need to work out.x2 − 3x
- Replace each letter with its value. Brackets keep a negative number together, so its sign is not lost.62 − 3 × 6
- Work out the calculation, in order. Powers first, then multiplication and division, then addition and subtraction.62 = 36; 3 × 6 = 18; 36 − 18 = 18
- Write the answer. Check that it is the value of the expression for the values given.18
Check. Check by replacing the letters again and working out the sum a second time. ✓
Watch out. Put brackets around each value before you multiply or square. Without brackets, −3 squared looks like −(3 squared).
Method. Replace each letter with its value, using brackets. Then work out the answer, following the order of operations.
- Write the expression or formula. This is what we need to work out.v = u + at
- Replace each letter with its value. Brackets keep a negative number together, so its sign is not lost.4 + 3 × 6
- Work out the calculation, in order. Powers first, then multiplication and division, then addition and subtraction.3 × 6 = 18; 4 + 18 = 22
- Write the answer. Check that it is the value of the expression for the values given.22
Check. Check by replacing the letters again and working out the sum a second time. ✓
Watch out. Put brackets around each value before you multiply or square. Without brackets, −3 squared looks like −(3 squared).
Method. Replace each letter with its value, using brackets. Then work out the answer, following the order of operations.
- Write the expression or formula. This is what we need to work out.3(p − 5)
- Replace each letter with its value. Brackets keep a negative number together, so its sign is not lost.3 × ((−2) − 5)
- Work out the calculation, in order. Powers first, then multiplication and division, then addition and subtraction.(−2) − 5 = −7; 3 × (−7) = −21
- Write the answer. Check that it is the value of the expression for the values given.−21
Check. Check by replacing the letters again and working out the sum a second time. ✓
Watch out. Put brackets around each value before you multiply or square. Without brackets, −3 squared looks like −(3 squared).
Method. Use the shorter notation rules: leave out the times sign, put the number first, and write repeated letters as powers.
- Work out the cost of the pens. n pens at 35p each cost 35 × n.35n
- Add the pencil. The pencil costs 20p, so add 20.35n + 20
Check. If n = 2, the total is 90p, and 35 × 2 + 20 = 90. ✓
Watch out. Write 35n, not n35.
Method. Replace each letter with its value, using brackets. Then work out the answer, following the order of operations.
- Write the expression or formula. This is what we need to work out.x2 − 2x
- Replace each letter with its value. Brackets keep a negative number together, so its sign is not lost.(−4)2 − 2 × (−4)
- Work out the calculation, in order. Powers first, then multiplication and division, then addition and subtraction.(−4)2 = 16; 2 × (−4) = −8; 16 − (−8) = 24
- Write the answer. Check that it is the value of the expression for the values given.24
Check. Check by replacing the letters again and working out the sum a second time. ✓
Watch out. Put brackets around each value before you multiply or square. Without brackets, −3 squared looks like −(3 squared).
Method. Replace each letter with its value, using brackets. Then work out the answer, following the order of operations.
- Write the expression or formula. This is what we need to work out.P = 2(l + w)
- Replace each letter with its value. Brackets keep a negative number together, so its sign is not lost.2 × (8.5 + 3.5)
- Work out the calculation, in order. Powers first, then multiplication and division, then addition and subtraction.8.5 + 3.5 = 12; 2 × 12 = 24
- Write the answer. Check that it is the value of the expression for the values given.24
Check. Check by replacing the letters again and working out the sum a second time. ✓
Watch out. Put brackets around each value before you multiply or square. Without brackets, −3 squared looks like −(3 squared).
Method. Substitute a = 3 into both expressions, and compare the answers.
- Work out 2(a + 1). The bracket is worked out first, then multiplied by 2.2(3 + 1) = 2 × 4 = 8
- Work out 2a + 1. Multiply 2 by 3 first, then add 1.2 × 3 + 1 = 7
- Compare. The answers are different, so the expressions are not the same.8 is not 7
Check. The 2 multiplies both the a and the 1 in the bracket: 2(a + 1) = 2a + 2. ✓
Watch out. Brackets change the order of working. A number outside a bracket multiplies everything inside it.