Quadratics

Solving by factorising

A quadratic $ax^2 + bx + c = 0$ may factorise into two brackets. If the product is zero, one of the factors is zero:

$$x^2 - 2x - 15 = 0 \;\Rightarrow\; (x - 5)(x + 3) = 0 \;\Rightarrow\; x = 5 \text{ or } x = -3$$