Law of Cosines

Theorem
Let $\triangle ABC$ be a triangle whose sides $a, b, c$ are such that $a$ is opposite $A$, $b$ is opposite $B$ and $c$ is opposite $C$.

Then $c^2 = a^2 + b^2 - 2ab \cos C$.

Historical Note
This result is known in France as Théorème d'Al-Kashi (Al-Kashi's Theorem) after, who is believed to have first discovered it.