Prosthaphaeresis Formulas/Cosine plus Cosine

Theorem

 * $\cos \alpha + \cos \beta = 2 \map \cos {\dfrac {\alpha + \beta} 2} \map \cos {\dfrac {\alpha - \beta} 2}$

where $\cos$ denotes cosine.

Also see

 * Prosthaphaeresis Formula for Sine plus Sine
 * Prosthaphaeresis Formula for Sine minus Sine
 * Prosthaphaeresis Formula for Cosine minus Cosine