Euler's Equation 
Written by Bryce Ringwood 
Please review the section on complex numbers, or you might get a bit lost. Here's Eulers equation: \color \orange {e^{ix} = cos(x) + i sin(x)}
Its easy to prove, we simply assume that $e^{ix}$ is a complex number, and we express it in polar coordinates: e^{ix}={r(cos(\theta) + i sin(\theta))}
Now we differentiate both sides with respect to x:
\displaystyle {ie^{ix}=(cos(\theta ) + i sin(\theta )){dr\over dx} +r (sin(\theta ) + i cos(\theta)){d{\theta }\over dx}} so:
\displaystyle {ir({(cos(\theta) + i sin(\theta))} = (cos(\theta)+ i sin(\theta )){dr\over dx} +r (sin(\theta ) + i cos(\theta)){d{\theta }\over dx}}
(We use the product rule to arrive at the above result  see previous article) If we equate real and imaginary parts: \displaystyle{ircos(\theta)=isin(\theta){dr\over dx }+ircos(\theta){d\theta \over dx}}
\displaystyle{rsin(\theta )= cos(\theta ){dr\over dx}rsin(\theta){d\theta \over dx}} These equations are satisfied if:
\displaystyle{{dr\over dx}=0} and \displaystyle {{{d\theta}\over dx} = 1}
This tells us the radius is a constant and that $\theta = x$  which we couldn't just assume. When $x=0$, then $cos(0)=1$, so $r=1$, since $e^{0i}=1$. This shows the theorem to be true. The important thing is that we can express a complex number as $re^{i\theta}$, which we need to do for impedance calculations. Aknowlegements to Wikpedia for bringing this method to my attention. I was going to do it another way, but I prefer this. FalloutHere are some interesting relationships that result from fiddling about with the above equation. They are all quite easy to derive: $\displaystyle{{e^{i\pi }}= 1}$ $\displaystyle{sin(\theta ) ={{e^{i\theta}  e^{i\theta }}\over 2i}}$ $\displaystyle{cos(\theta) ={{e^{i\theta } + e^{i\theta }}\over 2}}$
