Cosine Exponential Formulation

Theorem
For any complex number $x$,
 * $\cos x = \dfrac {e^{i x} + e^{-i x} } 2$

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

Also see

 * Sine Exponential Formulation
 * Tangent Exponential Formulation
 * Arccosine Logarithmic Formulation