Cosine of Integer Multiple of Argument/Formulation 2

Theorem
For $n \in \Z_{>0}$:

Proof
By De Moivre's Formula:
 * $\cos n \theta + i \sin n \theta = \paren {\cos \theta + i \sin \theta}^n$

As $n \in \Z_{>0}$, we use the Binomial Theorem on the, resulting in:


 * $\ds \cos n \theta + i \sin n \theta = \sum_{k \mathop \ge 0} \binom n k \paren {\cos^{n - k} \theta} \paren {i \sin \theta}^k$

When $k$ is even, the expression being summed is real.

Equating the real parts of both sides of the equation, replacing $k$ with $2 k$ to make $k$ even, gives: