While the most up-voted Pythagoras theorem proof [1] might be fun to see, it is rather the proof of the spectacular failure of our education - that nobody remembers anything taught in our school. The similarity of right triangles in a circle is proven using Pythagoras theorem in the first place!

Thankfully a better proof is present [2] which depends on distributive property and algebra.

1. http://math.stackexchange.com/a/733765/141120 2. http://math.stackexchange.com/a/734887/141120