Tangent Exponential Formulation/Formulation 1

Theorem
Let $z$ be a complex number.

Let $\tan z$ denote the tangent function and $i$ denote the imaginary unit: $i^2 = -1$.

Then:
 * $\tan z = i \dfrac {1 - e^{2 i z} } {1 + e^{2 i z} }$