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GCSE Maths · EdexcelChapter 7 · Algebra basics
GCSE Maths · Algebra Basics · Foundation · Lesson 7.3

GCSE Maths: Multiplying and dividing terms, and the laws of indices (Foundation)

Part of the GCSE Maths algebra basics chapter (Foundation). Chapter overview · Higher version of this lesson · whole chapter on one page · Download worksheet (PDF) · Answers (PDF)

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7.3

Multiplying and dividing terms, and the laws of indices

Edexcel 1MA1: A1, A4
Grade 3-6

You already know how to multiply numbers. Multiplying algebraic terms follows the same idea: multiply the numbers, then deal with the letters.

What is x × x? Picture a square with sides of length x. Its area is x × x, which we write as x2 (“x squared”). The little 2 is called the index or power. It says how many copies of the letter are multiplied together.

Area of a square with side xxx²x
A square with side x has area x × x = x2.

Multiplying terms

3a × 4b means 3 × a × 4 × b. Multiplication can be done in any order, so we can swap the numbers round: 3 × 4 × a × b = 12ab. When the same letter appears in both terms, count the copies. 5x × 2x = 10 × x × x = 10x2.

The laws of indices

Writing the letters out in full shows why these rules work. For example x3 × x2 = (x × x × x) × (x × x), which is five xs multiplied together, so it is x5.

LawExampleIn words
am × an = am+nx3 × x4 = x7multiplying: add the powers
am ÷ an = am−nx5 ÷ x2 = x3dividing: subtract the powers
(am)n = amn(x2)3 = x6a power of a power: multiply the powers
a1 = ay = y1a letter on its own has power 1
a0 = 1x4 ÷ x4 = x0 = 1anything (except 0) to the power 0 is 1

The number parts of the terms are just multiplied or divided as usual. Only the letter parts use the index laws. And when there is a power outside a bracket, it applies to everything in the bracket: (2x3)2 = 22 × (x3)2 = 4x6.

You have met these laws with numbers in Chapter 2 (for example 23 × 24 = 27). Here the base is a letter, but nothing else changes.

Dividing terms

To divide terms, divide the numbers and then subtract the powers of each letter. It is the same as cancelling. 12x5 ÷ 3x2 = 12x53x2 = 4x3. You can write a division of terms as a fraction and cancel the numbers and the letters.

Key ideas

  • Multiply terms: multiply the numbers, and add the powers of each letter. 3x2 × 4x3 = 12x5.
  • Divide terms: divide the numbers, and subtract the powers. 12x5 ÷ 3x2 = 4x3.
  • Power of a power: multiply the powers. (x2)3 = x6. A power outside a bracket applies to the number and every letter inside.
  • a1 = a and a0 = 1.
  • The laws only work when the letters (the bases) are the same.
Worked example 7.3.1
Simplify 2a2b × 3ab3.

Key words. The power or index (plural indices) tells you how many copies of the letter are multiplied together: x3 = x × x × x. The coefficient is the number in front of the letters.

Method. To multiply terms, multiply the numbers and add the powers of each letter. To divide, divide the numbers and subtract the powers. A power outside a bracket multiplies every power inside.

  1. Write the question. We are going to simplify one product or quotient of terms.
    2a2b × 3ab3
  2. Multiply the terms: first the numbers, then each letter. Multiply the numbers (coefficients) together. For each letter, add the powers, because x2 × x3 = (x × x) × (x × x × x) = x5 (the law am × an = am+n). A letter written on its own has power 1.
    Numbers: 2 × 3 = 6
    a: add the powers 2 + 1 = 3
    b: add the powers 1 + 3 = 4
    So the top is 6a3b4
  3. Write the answer. Put the number first, then the letters.
    6a3b4

Check. Test with numbers: put a = 2, b = 3 into the question and into the answer. The question gives 3888 and the answer gives 3888. They are the same. ✓

Watch out. Add the powers when you multiply (x2 × x3 = x5, not x6). Multiply the powers when there is a power of a power. And only the powers change: 3x × 2x = 6x2, the 3 and 2 are multiplied, not added.

Worked example 7.3.2
Simplify 5x × 2x.

Key words. The power or index (plural indices) tells you how many copies of the letter are multiplied together: x3 = x × x × x. The coefficient is the number in front of the letters.

Method. To multiply terms, multiply the numbers and add the powers of each letter. To divide, divide the numbers and subtract the powers. A power outside a bracket multiplies every power inside.

  1. Write the question. We are going to simplify one product or quotient of terms.
    5x × 2x
  2. Multiply the terms: first the numbers, then each letter. Multiply the numbers (coefficients) together. For each letter, add the powers, because x2 × x3 = (x × x) × (x × x × x) = x5 (the law am × an = am+n). A letter written on its own has power 1.
    Numbers: 5 × 2 = 10
    x: add the powers 1 + 1 = 2
    So the top is 10x2
  3. Write the answer. Put the number first, then the letters.
    10x2

Check. Test with numbers: put x = 2 into the question and into the answer. The question gives 40 and the answer gives 40. They are the same. ✓

Watch out. Add the powers when you multiply (x2 × x3 = x5, not x6). Multiply the powers when there is a power of a power. And only the powers change: 3x × 2x = 6x2, the 3 and 2 are multiplied, not added.

Worked example 7.3.3
Simplify 15a3b25ab.

Key words. The power or index (plural indices) tells you how many copies of the letter are multiplied together: x3 = x × x × x. The coefficient is the number in front of the letters.

Method. To multiply terms, multiply the numbers and add the powers of each letter. To divide, divide the numbers and subtract the powers. A power outside a bracket multiplies every power inside.

  1. Write the question. We are going to simplify one product or quotient of terms.
    15a3b25ab
  2. Now divide: divide the numbers, and for each letter subtract the powers. Dividing by a term is the same as cancelling. x5 ÷ x2 = (x × x × x × x × x) ÷ (x × x) = x3 (the law am ÷ an = am−n). Divide the numbers as you would divide any numbers.
    Numbers: 15 ÷ 5 = 3
    a: subtract the powers 3 − 1 = 2
    b: subtract the powers 2 − 1 = 1
  3. Write the answer. Put the number first, then the letters.
    3a2b

Check. Test with numbers: put a = 2, b = 3 into the question and into the answer. The question gives 36 and the answer gives 36. They are the same. ✓

Watch out. Add the powers when you multiply (x2 × x3 = x5, not x6). Multiply the powers when there is a power of a power. And only the powers change: 3x × 2x = 6x2, the 3 and 2 are multiplied, not added.

Worked example 7.3.4
Simplify (3ab2)3.

Key words. The power or index (plural indices) tells you how many copies of the letter are multiplied together: x3 = x × x × x. The coefficient is the number in front of the letters.

Method. To multiply terms, multiply the numbers and add the powers of each letter. To divide, divide the numbers and subtract the powers. A power outside a bracket multiplies every power inside.

  1. Write the question. We are going to simplify one product or quotient of terms.
    (3ab2)3
  2. Deal with any power outside a bracket first. A power outside a bracket applies to everything inside it: the number and every letter. For example (2x3)2 means 2x3 × 2x3. The number is raised to the power, and each letter's own power is multiplied by it (the law (am)n = amn).
    (3ab2)3 → number: 33 = 27; a: 1 × 3 = 3; b: 2 × 3 = 6 → 27a3b6
  3. Write the answer. Put the number first, then the letters.
    27a3b6

Check. Test with numbers: put a = 2, b = 3 into the question and into the answer. The question gives 157464 and the answer gives 157464. They are the same. ✓

Watch out. Add the powers when you multiply (x2 × x3 = x5, not x6). Multiply the powers when there is a power of a power. And only the powers change: 3x × 2x = 6x2, the 3 and 2 are multiplied, not added.

Watch out

Only the powers are added when you multiply. 3x × 2x = 6x2: the numbers are multiplied (3 × 2 = 6), they are not added, and the power of x goes from 1 + 1 = 2. Also x3 × x4 is x7, not x12. And x2 + x3 cannot be simplified at all: the laws are for multiplying and dividing, not for adding.

Exercise 7C  Multiplying, dividing and indices

Non-calculator
1
FSimplify each product.
(a)4x × 3
(b)2a × 5b
(c)x × x
(d)3y × 2y
(e)5m × m2
(f)ab × ab
2
FWrite each as a single power.
(a)x3 × x4
(b)a5 ÷ a2
(c)(y2)3
(d)p × p × p
(e)m6 ÷ m
(f)k8 × k
3
FSimplify each product.
(a)3x2 × 2x4
(b)4a3 × 5a
(c)6b × b2
(d)2y3 × 7y2
4
FSimplify each division.
(a)12x ÷ 3
(b)20a2 ÷ 4a
(c)15y3 ÷ 5y
(d)18p4 ÷ 6p2
(e)8m ÷ 2m
(f)9t2 ÷ 3t2
5
FSimplify each product.
(a)3ab × 2a
(b)4xy × 5x
(c)2pq × 3pq
(d)a2b × ab2
6
PSThe rectangle has length 5x2 cm and width 3x cm. Write a simplified expression for its area.
5x²3x
7
PSA square has sides of length 4a. Write a simplified expression for its area.
8
PSA cuboid has length 2x cm, width 3x cm and height x2 cm. Write a simplified expression for its volume.
9
RAli says that 2x × 3x = 6x. Use x = 2 to test his answer, and give the correct answer.
10
RLena writes 3x2 × 2x3 = 6x6. Find her mistake and write the correct answer.
11
FSimplify each expression.
(a)(x2)3
(b)(2a)3
(c)(3b2)2
(d)(5x)2
(e)(ab)4
(f)(2y3)4
12
FSimplify each fraction.
(a)6x23x
(b)10a3b5ab
(c)12xy24y
(d)15p3q23pq
13
PSFind the missing power in each statement.
(a)x? × x4 = x9
(b)x7 ÷ x? = x2
(c)(x?)3 = x12
(d)3x? × 2x2 = 6x5
14
RShow that (2x2)34x4 = 2x2.
15
FWork out the value of each. Explain your answers.
(a)50
(b)y0
(c)6x0
(d)(3a)0
16
PSThe area of a rectangle is 12x3 cm2. Its length is 4x cm. Write an expression for its width.

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