**Expanding** **Brackets**

Brackets should be expanded in the following ways:

For an expression of the form a(b + c), the expanded version is ab + ac, i.e.,
multiply the term outside the bracket by everything inside the bracket (e.g.
2x(x + 3) = 2x² + 6x [remember x × x is x²])

For an expression of the form (a + b)(c + d), the expanded version is ac + ad +
bc + bd, in other words everything in the first bracket should be multiplied by
everything in the second.

*Example*:

Expand (2x + 3)(x - 1):

(2x + 3)(x - 1)

= 2x² - 2x + 3x - 3

= __2x² + x – 3
__

**Factorising**

Factorising is the reverse of expanding brackets, so it is putting 2x² + x - 3
into the form (2x + 3)(x - 1). This is an important way of solving quadratic
equations.

The first step of factorising an expression is to 'take out' any common factors
which the terms have. So if you were asked to factorise x² + x, since x goes
into both terms, you would write x(x + 1) .

**Factorising Quadratics**

There is no simple method of factorising a quadratic expression. One way,
however, is as follows:

*Example*:

Factorise 12y² - 20y + 3

12y² - 18y - 2y + 3 [here the 20y has been split up into two
numbers whose multiple is 36. 36 was chosen because this is the product of 12
and 3, the other two numbers].

The first two terms, 12y² and -18y both divide by 6y, so 'take out' this factor
of 6y.

6y(2y - 3) - 2y + 3 [we can do this because 6y(2y - 3) is the same as 12y² -
18y]

Now, make the last two expressions look like the expression in the bracket:

6y(2y - 3) -1(2y - 3)

The answer is __(2y - 3)(6y - 1)__

*Example*:

Factorise x² + 2x - 8

We need to split the 2x into two numbers which multiply to give -8. This has to
be 4 and -2.

x² + 4x - 2x - 8

x(x + 4) - 2x - 8

x(x + 4)- 2(x + 4)

__(x + 4)(x - 2)__

Once you work out what is going on, this method makes factorising any expression
easy. It is worth studying these examples further if you do not understand what
is happening. Unfortunately, the only other method of factorising is by trial
and error.

**The Difference of Two Squares**

If you are asked to factorise an expression which is one square number minus
another, you can factorise it immediately. This is because a² - b² = (a + b)(a
- b) .

*Example*:

Factorise 25 - x²

= __(5 + x)(5 - x)__ [imagine that a = 5 and b = x]

(c) Matthew Pinkney