De Moivre's Formula

Theorem

 * $$\left(\cos x+i\sin x\right)^n=\cos\left(nx\right)+i\sin\left(nx\right)\,$$, for $$\begin{cases}n=1,2,3,\ldots \\ x\in\R\end{cases}$$

Also known as De Moivre's Theorem.

Proof
The formula can be derived from Euler's formula,


 * $$e^{ix} = \cos x + i\sin x\,$$

and laws of exponentials,


 * $$\left( e^{ix} \right)^n = e^{inx} .\,$$

Then, by Euler's formula,


 * $$e^{i(nx)} = \cos(nx) + i\sin(nx)\,$$.

Base Case (n=1)
True since $$(\cos{x}+i\sin{x})^1=\cos(1x)+i\sin(1x)$$

Induction Hypothesis(n=k)
Assume true for $$n=k$$ to get $$ (\cos{x}+i\sin{x})^k=\cos(kx) + i\sin(kx)$$

Show true for n=k+1
$$ $$ $$ $$ $$

So true for $$n=k+1$$, hence true for all $$n=1,2,3,\ldots$$ by mathematical induction.