Cosine Exponential Formulation

Theorem
For any complex number $z \in \C$:
 * $\cos z = \dfrac {\exp \paren {i z} + \exp \paren {-i z} } 2$

where:
 * $\exp z$ denotes the exponential function
 * $\cos z$ denotes the complex cosine function
 * $i$ denotes the inaginary unit.

Real Domain
This result is often presented and proved separately for arguments in the real domain:

Also presented as
This result can also be presented as:
 * $\cos z = \dfrac 1 2 \paren {e^{-i z} + e^{i z} }$

Also see

 * Sine Exponential Formulation
 * Tangent Exponential Formulation
 * Cotangent Exponential Formulation
 * Secant Exponential Formulation
 * Cosecant Exponential Formulation


 * Arccosine Logarithmic Formulation