Cosine Exponential Formulation/Proof 2

Theorem
For any complex number $x$:


 * $\cos x = \dfrac {e^{i x} + e^{-i x} } 2$

where $\cos x$ is the cosine and $i^2 = -1$.

Proof
Recall Euler's Formula:


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

Then, starting from the RHS: