
You have probably learnt how to factorise a quadratic expression: for example, x2 - 2x - 8 factorises to give (x - 4)(x + 2). This is useful because it enables us to solve the equation x2 - 2x - 8 = 0. How? Because if we use the factorised form we get (x - 4)(x + 2) = 0, and if two numbers multiply to give zero then one of the number must be zero. So either x - 4 = 0, ...