Pythagoras's Theorem

Theorem
Let $\triangle ABC$ be a right triangle with $c$ as the hypotenuse.

Then:
 * $a^2 + b^2 = c^2$

Also known as
Pythagoras's theorem was known to the Pythagoreans as the Theorem of the Bride, from its numerological significance.

Also see

 * Pythagorean Theorem (Hilbert Space), a generalisation to the context of Hilbert spaces.