I am looking for examples of geometric proofs of theorems in fields other than euclidean geometry. The more surprising the fact that a geometric proof is possible, the better. As two examples: -From Topics in the Theory of Numbers by Erdos, the proof that square root c is irrational if c is not a perfect square -Isaac Barrow's proof of the Fundamental Theorem of Calculus
Here are images for a search for "proofs without words"
(I hope this link only answer is OK for community wiki.)