Cosine Exponential Formulation

Theorem
For any complex number $x$,
 * $ \displaystyle \cos x = \frac 1 2 \left({ e^{-i x} + e^{i x} }\right)$

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

Also see

 * Sine Exponential Formulation
 * Tangent Exponential Formulation