Delboeuf Proof

Loomis 56

Part II, Proof 56, page 134 from E.S.Loomis, The Pythagorean Proposition (1940)

These are really two squares one atop the other, ready to animate. This black monolyth is hidding all the parts of this proof inside itself. This proof, and the two that follow, may be called "surround" proofs that pad out the two out-side squares with triangles. What follows below is a three picture "animation." These pictures show the surrounding triangles growing up to 45 degrees.