Euler's Formula/Real Domain/Corollary

Corollary to Euler's Formula: Real Domain
Let $\theta \in \R$ be a real number.

Then:
 * $e^{i \theta} = \cos \theta - i \sin \theta$

where:
 * $e^{i \theta}$ denotes the complex exponential function
 * $\cos \theta$ denotes the real cosine function
 * $\sin \theta$ denotes the real sine function
 * $i$ denotes the imaginary unit.