Well I personally liked the Euler's Theorem when I first saw, and I feel one can easily understand it at the High-School level.

Well I personally liked the Euler's Theorem when I first saw, and I feel one can easily understand it at the High-School level.

There is a Youtube video with William Dunham's lecture on Euler which contains this theorem at 32:30 : https://www.youtube.com/watch?v=fEWj93XjON0