My favorite theorem is the Pythagorean Theorem. It is much better than EVAN'S FAVORITE THEOREM_MA375Fall2008walther.
(No seriously, I chose it before looking at his page) --
It's one of those theorems that is surprising at first. When I saw the proof of it I was pretty amazed.File:Http://upload.wikimedia.org/wikipedia/commons/9/9a/Pythagorean proof (1).svg
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