Secant in terms of Hyperbolic Secant

Theorem

 * $\operatorname{sech} \left({ix}\right) = \sec x $

where $\sec$ is the secant function, $\operatorname{sech}$ is the hyperbolic secant, and $i^2=-1$.

Also see

 * Sine in terms of Hyperbolic Sine
 * Cosine in terms of Hyperbolic Cosine
 * Tangent in terms of Hyperbolic Tangent
 * Cotangent in terms of Hyperbolic Cotangent
 * Cosecant in terms of Hyperbolic Cosecant