1
00:00:00,000 --> 00:00:03,000
Four identical right triangles form a frame around two squares.

2
00:00:06,000 --> 00:00:09,000
The four triangles have total area 4 times ab over 2.

3
00:00:09,000 --> 00:00:12,000
What's left? A square of area a² and a square of area b².

4
00:00:15,033 --> 00:00:18,033
Now rearrange. Does the large square still fit perfectly?