# GCSE Factorising revision

A quick, one-off masterclass in how to put things into brackets today - six methods of factorising you need to know to do well at GCSE maths.

## (1) Common number

$3a + 6$

• two terms (letter and number, no squares)
• you can divide them both by 3
• $3 \times a = 3a$
• $3 \times 2 = 6$
• $3a + b = 3(a + 2)$

### Try these:

$9b + 12$
$12c - 8$
$90d + 15$
$7e - 14$

## (2) Common letter

$x^2 - 4x$

• two terms (letter-squared and letter)
• you can divide them both by $x$
• $x \times x = x^2$
• $x \times -4 = -4x$
• $x^2 - 4x = x(x-4)$

### Try these:

$b^2 + 9b$
$c^2 - 2c$
$d^2 + 10d$

## (3) Common number and letter

$6x^2 - 4x$

• two terms (letter-squared and letter)
• you can divide them both by $2x$
• $2x \times 3x = 6x^2$
• $2x \times -2 = -4x$
• $6x^2 - 4x = 2x(3x-2)$

### Try these:

$2x^2 + 8x$
$3x^2 - 6x$
$4x^2 - 10x$

## (4) Difference of two squares

$4x^2 - 25$

• two terms (both squares)
• Use $(a+b)(a-b) = a^2 - b^2$
• $4x^2 = (2x)^2$, so $a = 2x$
• $25 = 5^2$, so $b = 5$
• $4x^2 - 25 = (2x -5)(2x + 5)$

### Try these:

$x^2 - 9$
$9x^2 - 1$
$4a^2 - 9b^2$

$x^2 - 2x - 15$

• Three terms (letter-squared, letter and number)
• Look for two numbers $p$ and $q$ such that:
• $pq = -15$; and
• $p+q = -2$. (Adding things in the middle would be the end of the Times!)
• Not too many possibilities that multiply to -15: 1 and -15, 3 and -5, 5 and -3, 15 and -1. Only 3 and -5 work.
• $x^2 - 2x - 15 = (x+3)(x-5)$

### Try these:

$x^2 + 3x + 2$
$x^2 - 3x + 2$
$x^2 + 13x + 36$
$x^2 + 2x - 35$

## (6) Quadratic with a number in front

$4x^2 + 8x + 3$

• Three terms (letter-squared, letter and number)
• More difficult! Magic number is $4 \times 3 = 12$
• Want two numbers $p$ and $q$ such that:
• $pq = 12$
• $p + q = 8$
• 2 and 6 work!
• Split up $8x$ as $2x + 6x$ and write out: $4x^2 + 2x + 6x + 3$
• Factorise first half: $2x(2x + 1)$
• Factorise second half: $3(2x + 1)$
• Combine: $(2x+3)(2x+1)$ - phew!

### Try these:

* $2x^2 + 3x + 1$
* $3y^2 + 8y - 3$
* $4z^2 + 5z + 1$

* Edited 2016-05-08 to correct wrong letters in last two questions. Thanks, Rosie, for pointing out my error. ## Colin

Colin is a Weymouth maths tutor, author of several Maths For Dummies books and A-level maths guides. He started Flying Colours Maths in 2008. He lives with an espresso pot and nothing to prove.

### 3 comments on “GCSE Factorising revision”

• ##### Rosie

how would you factorise the very last two questions of (6) when the letters are different?

• ##### Colin

Yikes! You would sneakily edit the quiz to correct the questions! Good spot, thank you. Fixed now.

• ##### Naomi Boitumelo

Dear Colin
thanks very much for your help,i do understand better than before.keep on posting

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