# Cosine rule

The cosine rule states that if $a$, $b$ and $c$ are the side lengths of a triangle, and $A$ is the angle opposite side $a$, then $a^2 = b^2 + c^2 - 2bc\cos A.$

This is a generalisation of Pythagoras’ Theorem, as if $A=90^\circ$, then $\cos A=0$.