Cosine in terms of Hyperbolic Cosine

Theorem

 * $\cosh \left({ix}\right) = \cos x$

where $\cos$ is the cosine, $\cosh$ is the hyperbolic cosine, and $i^2=-1$.

Also see

 * Hyperbolic Sine of Imaginary Number
 * Hyperbolic Tangent of Imaginary Number
 * Hyperbolic Cotangent of Imaginary Number
 * Hyperbolic Secant of Imaginary Number
 * Hyperbolic Cosecant of Imaginary Number