Factorising

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In order to access this I need to be confident with:

Expanding brackets Expand and simplify

Factors, multiples, powers and roots

Algebraic expressions

Adding and subtracting negative numbers

Multiplying and dividing negative numbers

Highest common factor (HCF)

Laws of indices

Square numbers and square roots

This topic is relevant for:

GCSE Maths Algebra

Factorising

Here is everything you need to know about factorising for GCSE maths (Edexcel, AQA and OCR). You’ll learn the essentials of factorising expressions and factorising quadratics including factorising into single brackets and double brackets.

Look out for the factorising worksheets and exam questions at the end.

What is factorising

Factorising is the reverse process of expanding brackets. To factorise an algebraic expression means to put it in brackets by taking out the common factors.

The simplest way of factorising is

Find the highest common factor of each of the terms in the expression.
Write the highest common factor (HCF) in front of any brackets
Fill in each term in the brackets by multiplying out.

What is factorising?

The reverse process of expanding brackets.

To factorise an algebraic expression means to put it in brackets by taking out the common factors.

E.g. Factorising

3x + 6 = 3(x + 2)

However there are different ways to factorise different types of algebraic expressions; we will learn about them all here.

How to factorise expressions

To factorise algebraic expressions there are three basic methods. When you are factorising quadratics you will usually use the double brackets or difference of two squares method.

1. Factorising single brackets

Example of factorising an algebraic expression:

3x + 6 = 3(x + 2)

Remember: 3x+6 is known as a binomial because it is an expression with two terms

2. Factorising double brackets

a) When factorising quadratic expressions in the form x² + bx + c

x² + 6x + 5 = (x + 5)(x + 1)

b) When factorising quadratic expressions in the form ax² + bx + c

2x² + 5x + 3 = (2x + 3)(x + 1)

Remember:
Expressions with three terms like x²+ 6x + 5 and 2x²+ 5x + 3 are known as a trinomials.

3. Differences of two squares

Using the difference of two squares:

4x² – 16 = (2x – 4)(2x + 4)

What are the 3 standard ways to factorise algebraic expressions?

3 ways to factorise algebraic expressions:

Factorising to single brackets
Factorising to double brackets
Difference of two squares

Factorisation methods

Each method of factorising or factoring expressions is summarised below. For detailed examples, practice questions and worksheets on each one follow the links to the step by step guides.

1. Factorising single brackets

Factorising example using single brackets

To fully factorise:

\[\color{#00BC89}3x + \color{#7C4DFF}6\]

Find the highest common factor (HCF) of the numbers 3 (the coefficient of x) and 6 (the constant).

Factors of 3:
1, 3

Factors of 6:
1, 6
2, 3

Top tip:
Writing the factor pairs makes it easier to list all the factors

The highest common factor (HCF) of 3x and 6 is 3

2 Write the highest common factor (HCF) at the front of the single bracket.

\[\color{#FF9100}3(\quad+\quad)\]

3 Fill in each term in the bracket by multiplying out.

What do I need to multiply 3 by to give me 3x?

\[\color{#FF9100}3 \times \color{#62F030}x = \color{#00BC89}3 x\]

What do I need to multiply 3 by to give me 6?

\[\color{#FF9100}3 \times \color{#92009E}2 = \color{#7C4DFF}6\]

\[\color{#FF9100}3(\color{#62F030}x + \color{#92009E}2)\]

We can check the answer by multiplying out the bracket!

\[3(x+2)=3 x+6\]

Step by step guide: Factorising single brackets

2a) Factorising quadratics into double brackets: x² + bx + c

Factorising example for quadratic expressions in the form x² + bx + c

To fully factorise:

\[x^2 + \color{#00bc89}6x + \color{#7C4DFF}5\]

Write out the factor pairs of the last number (5)

Factors of 5:
1, 5

2 Find a pair of factors that + to give the middle number (6) and ✕ to give the last number (5).

1 + 5 = 6 ✔

1 ✕ 5 = 5 ✔

3 Write two brackets and put the variable at the start of each one.

\[(x\qquad )(x\qquad)\]

4 Write one factor in the first bracket and the other factor in the second bracket. The order isn’t important, the signs of the factors are.

\[(x+\color{#FF9100}1)(x+\color{#FF9100}5)\]

2b) Factorising quadratics into double brackets: ax² + bx + c

Factorising example for quadratic expressions in the form ax² + bx + c

To fully factorise:

\[\color{#FE47EC}2x^2 + \color{#00bc89}5x \color{#7C4DFF}{+3}\]

Multiply the end numbers together (2 and 3) then write out the factor pairs of this new number in order

Factors of 6:
1, 5
2, 3

2 We need a pair of factors that + to give the middle number (5) and ✕ to give this new number (6)

2 + 3 = 5 ✔

2 ✕ 3 = 6 ✔

3 Rewrite the original expression, this time splitting the middle term into the two factors we found in step 2.

\[2 x^{2}\color{#FF9100}{+2 x+3 x}+3\]

4 Split the equation down the middle and fully factorise each half.

\[\color{#398CDA}{2 x}\color{#62F030}{(x+1)}\color{#398CDA}{+3}\color{#62F030}{(x+1)}\]

5 Factorise the whole expression by bringing whatever is in the bracket to the front and writing the two other terms in the other bracket.

\[(\color{#398CDA}{2x+ 3})\color{#62F030}{(x + 1)}\]

Step by step guide: Factorising quadratics

3. Difference of two squares

Factorising example using difference of two squares:

To fully factorise:

\[\color{#FE47EC}4x^2 -\color{#7C4DFF}{ 9}\]

Write down 2 brackets.

\[(\qquad)(\qquad)\]

2 Square root the first term and write it on the left-hand side of both brackets.

\[\sqrt{\color{#FE47EC}{4 x^2}}=\color{#FE47EC}{2 x}\]

\[(\color{#FE47EC}{2x}\qquad)(\color{#FE47EC}{2x}\qquad)\]

3 Square root the last term and write it on the right-hand side of both brackets.

\[\sqrt{\color{#7C4DFF}9}=\pm\color{#7C4DFF}3\]

\[(\color{#FE47EC}{2x}\quad\color{#7C4DFF}3)(\color{#FE47EC}{2x}\quad\color{#7C4DFF}3)\]

4 Put + in the middle of one bracket and - in the middle of the other (the order doesn’t matter).

\[(\color{#FE47EC}{2x}+\color{#7C4DFF}3)(\color{#FE47EC}{2x}-\color{#7C4DFF}3)\]

Step by step guide: Difference of two squares

Practice factorising questions (mixed)

5(2-y)

10(1-y)

5(2-5y)

5(2+y)

The highest common factor of $10$ and $5$ is $5$ . Therefore, we can divide the original expression by $5$ , which means the bracket must contain $2-y$ .

4(5x^{2}-2x)

4x(5x-2)

2x(10x-4)

4x^{2}(5x-2)

The highest common factor of $20x^{2}$ and $8x$ is $4x$ . Therefore, we can divide the original expression by $4x$ , which means the bracket must contain $5x-2$ .

(x+2)(x-3)

(x-2)(x-3)

(x-2)(x+3)

(x+1)(x-6)

In this case, we are looking for numbers that multiply to make $-6$ and add to make $-1$ . By considering the factor pairs of $-6$ , we conclude that we can use $+2$ and $-3$ .

(2x+2)(2x-3)

2(x^{2}-2x-3)

(2x+2)(x-3)

2(x+1)(x-3)

The original expression can be divided by $2$ , and then written as a product of two brackets.

2{x}^2-4x-6

Divide each term by $2$ , and rewrite.

2(x^{2}-2x-3)

To factorise the quadratic expression, we are looking for numbers that multiply to $-3$ and sum to $-2$ . By considering factor pairs, we conclude that we need to use $+1$ and $-3$ .

2(x+1)(x-3)

(x+3)(x+3)

(x+1)(x-9)

(x+3)(x-3)

(x+1)(x-3)

This is a special case (difference of two squares), which means we can take square roots of the coefficient of $x$ and the constant term, then write one bracket with a $+$ sign and the other bracket with a $-$ sign.

(3x+4)(3x-4)

(3x-4)(3x-4)

(9x+16)(x-1)

(3x+1)(3x-16)

Factorising GCSE questions (mixed)

1. Factorise: 9x − 18

Show answer

9(x − 2)

(1 mark)

2. Factorise fully: 16x² + 20xy

Show answer

4x(4x + 5y)

(2 marks)

3. Factorise fully: 3y² − 4y − 4

Show answer

(3y + 2)(y − 2)

(2 marks)

Factorising (mixed) worksheet

Get your free factorising worksheet of 20+ questions and answers. Includes reasoning and applied questions.

DOWNLOAD FREE

Learning checklist

You have now learnt how to:

Manipulate algebraic expressions by taking out common factors to factorise into a single bracket.
Factorise quadratic expressions of the form x² + bx + c
Factorise quadratic expressions of the form of the difference of two squares.
Factorise quadratic expressions of the form ax² + bx + c (H)

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