Show Me the Maths - Petra 

Hmm, it might be a NP hard problem to take the mathematician out of the maths and I believe that the International Mathematical Union (IMU) is about those mathematicians.

I don't think it's obvious but there's a very comprehensible proof for that identity. the proof I know, and I guess it's the most simple one, involves pretty much nothing but Taylor Series(it's a way to make a polinomial that approaches functions, in this case e^x, sinx and cosx. this is probably how you calculator computes them. if you wanna know also why is that true, there's a nice proof on wikipedia), and highschool level trigonometry and complex numbers. the core of the proof is using the taylor series of e^x, plugging ix instead of x, extracting the i with algebra and observing it's the taylor series of cosx, plus i times the taylor series of sinx. now you have the formula e^ix=cosx + isinx, plug π instead of x and you get -1.

why not?