Hyperbolic Cosine in terms of Cosine

Theorem

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

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

Also see

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