Law of Cosines/Right Triangle

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

Let $\triangle ABC$ be a right triangle such that $\angle A$ is right.

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

Proof
Let $\triangle ABC$ be a right triangle such that $\angle A$ is right.


 * CosineRule-Proof3-right.png

Hence the result.