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:
 * $c^2 = a^2 + b^2 - 2 a b \cos C$

Also see

 * Law of Sines
 * Law of Tangents