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 \(\cos A=0\).