Description: James Garfield's proof of the Pythagorean Theorem. Created by Sal Khan. What we're going to do in this video is study a proof of the Pythagorean theorem that was first discovered, or as far as we know first discovered, by James Garfield in 1876, and what's exciting about this is he was not a professional mathematician. You might know James Garfield as the 20th president of the United States.*Teacher Advisor is 100% free.