# Solve by the quadratic formula

Some quadratic equations cannot be solved by factorising and so an alternative method is the quadratic formula. It is based on a method called completing the square.

#### Rule to remember:

• $\text{Then it can be shown that} \hphantom{aaa} x=\dfrac{- b \pm \sqrt {{b^2} – 4ac}}{2a}$

This tutorial shows you when to use it, clues that are given in a question and common mistakes that are often made.

## How to Derive the Quadratic Formula

2017-11-04T17:03:51+00:00