English: Animation of a proof of Pythagoras' theorem, showing how by rearranging triangles the areas a² + b² and c² 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, a² + b², must equal the black area at the end, c². The angle of the triangles is arbitrary, therefore this works as a general proof. To be more explicit...

The situation at the start is:

Area_of_the_Outer_Square - (4 x Area_of_a_Triangle) = a² + b²

...and the situation at the end is:

Area_of_the_Outer_Square - (4 x Area_of_a_Triangle) = c²

The values on both left-hand sides of these two equations are exactly the same (only positions have changed) therefore the values on the right-hand sides must also be exactly the same: a² + b² = c².