# Cosine Rule The **cosine rule** extends [[Pythagoras' Theorem]] to all triangles. +-- {: .image} [[CosineRule.png:pic]] =-- With side lengths and angles as in the above diagram then: $$ c^2 = a^2 + b^2 - 2 a b \cos(C) $$ (and the same with other permutations of the letters)