Pythagorean theorem

The Pythagorean theorem states that, for a right triangles with side lengths a, b and hypotenuse c: 

a^2 + b^2 = c^2.

There are many proofs, but my favourite for its geometric simplicity is the following rearrangement of a square with side length a+b.

A proof by rearrangement of the Pythagorean theorem, based on moving four copies of the right triangle around a square with side length a+b