Prosthaphaeresis Formulas/Hyperbolic Cosine plus Hyperbolic Cosine

Theorem

 * $\cosh x + \cosh y = 2 \cosh \left({\dfrac {x + y} 2}\right) \cosh \left({\dfrac {x - y} 2}\right)$

where $\cosh$ denotes hyperbolic cosine.