Euler's identity
$ e^{j \pi} + 1 = 0, \,\! $
Euler's formula
$ e^{jx} = \cos x + i \sin x \,\! $
$ \cos x = \mathrm{Re}\{e^{ix}\} ={e^{ix} + e^{-ix} \over 2} $
$ \sin x = \mathrm{Im}\{e^{ix}\} ={e^{ix} - e^{-ix} \over 2i}. $
$ \cos(y) = {e^{-jy} + e^{jy} \over 2} $
