Animation of a proof of Pythagorean Theorem, showing how by rearranging triangles the areas a^2 + b^2 and c^2 can be shown to be the same. The area of the outer square never changes, and the total area of the four right triangles is the same at both the beginning and the end, therefore the black area at the beginning, a^2 + b^2, must equal the black area at the end, c^2. The angle of the triangles is arbitrary, therefore this works as a general proof.

Also, check this out: Math is Fun: “Pythagoras’ Theorem”