Euler's Formula
Basically, it is a mathematical formula in complex analysis in the form of:
$ e^{ix} = \cos x + i\sin x \! $
where i is the imaginary number, and x is any real number.
Interestingly, although it is named after the brilliant mathematician Leonhard Euler, he was not the first to prove this formula. It was Roger Cotes who had proven it for the very first time in 1714.
