# Geometric proofs outside euclidean geometry

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

• Also, I know this should probably be community wiki, but don't know how to make it one – Gotthold Apr 22 '17 at 12:27

There is a geometric (or at least, discrete-geometric) proof of Wilson's theorem, $(p - 1)! \equiv -1 \pmod p$ for a prime $p$.