Cosine of Difference

Corollary to Cosine of Sum

 * $\cos \paren {a - b} = \cos a \cos b + \sin a \sin b$

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

Proof
{{eqn | l = \cos \paren {a - b}     | r = \cos a \cos \paren {-b - \sin a \sin \paren {-b} | c = Cosine of Sum }}

Historical Note
This formula was proved by in about 1579.