Prosthaphaeresis Formulas/Cosine minus Cosine

Theorem

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

where $\sin$ denotes sine and $\cos$ denotes cosine.

Also see

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