Euler's Formula/Real Domain/Corollary

Corollary to Euler's Formula

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

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