Cosine of Difference

Corollary of Sine and Cosine of Sum

 * $\cos \left({a - b}\right) = \cos a \cos b + \sin a \sin b$


 * $\sin \left({a - b}\right) = \sin a \cos b - \cos a \sin b$

Proof
Similarly, we obtain:

Historical Note
These formulas were proved by François Viète in about 1579.