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Here is everything you need to know about factorising for GCSE maths (Edexcel, AQA and OCR). Youβll get an introduction to the essentials of factorising expressions and factorising quadratics including when how to use factorising with single brackets and factorising with double brackets.
Look out for the factorising worksheet and exam questions at the end.
Factorising is the reverse process of expanding brackets. To factorise an expression fully, means to put it in brackets by taking out the highest common factors.
The simplest way of factorising is:
However there are different ways to factorise different types of algebraic expressions; we will learn about them all here.
Get your free factorising worksheet of 20+ questions and answers. Includes reasoning and applied questions.
DOWNLOAD FREEGet your free factorising worksheet of 20+ questions and answers. Includes reasoning and applied questions.
DOWNLOAD FREETo 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:
Remember:
2. Factorising double brackets
a) When factorising quadratic expressions in the form
b) When factorising quadratic expressions in the form
Remember:
Expressions with three terms like
3. Differences of two squares
Using the difference of two squares:
Each method of factorising or factoring expressions is summarised below. For more detail, practice questions and worksheets on each factorisation example follow the links to the step by step guides.
Factorising example using single brackets
To factorise fully:
The highest common factor (HCF) of
2 Write the highest common factor (HCF) at the front of the single bracket.
3 Fill in each term in the bracket by multiplying out.
What do I need to multiply
What do I need to multiply
We can check the answer by multiplying out the bracket!
Step by step guide: Factorising single brackets
Factorising example for quadratic expressions in the form
To factorise fully:
Factors of 5:
1, 5
2 Find a pair of factors that + to give the middle number (
1 + 5 = 6 β 1 β 5 = 5 β
3 Write two brackets and put the variable at the start of each one.
4 Write one factor in the first bracket and the other factor in the second bracket. The order isnβt important, the signs of the factors are.
Factorising example for quadratic expressions in the form
To factorise fully:
Factors of 6:
1, 6
2, 3
2 We need a pair of factors that + to give the middle number (5) and β to give this new number (6)
3 Rewrite the original expression, this time splitting the middle term into the two factors we found in step 2.
4 Split the equation down the middle and factorise fully each half.
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.
Step by step guide: Factorising quadratics
See also: Quadratic equations
Factorising example using difference of two squares:
To factorise fully:
2 Square root the first term and write it on the left-hand side of both brackets.
3 Square root the last term and write it on the right-hand side of both brackets.
4 Put + in the middle of one bracket and – in the middle of the other (the order doesnβt matter).
Step by step guide: Difference of two squares
1. Fully factorise:
10-5y
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 .
2. Fully factorise:
20{x}^2-8xΒ
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 .
3. Fully factorise:
{x}^2-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 .
4. Fully factorise:
2{x}^2-4x-6
The original expression can be divided by 2 , and then written as a product of two brackets.
2{x}^2-4x-6Divide 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)5. Fully factorise:
{x}^2-9
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.
6. Fully factorise:
9{x}^2-16
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.
You have now learnt how to:
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