## Precalculus (6th Edition) Blitzer

Identity for $\cos \left( x+y \right)$ is expressed as $\cos x\cos y-\sin x\sin y$.
From the sum formula of cosines, the cosine of the sum of two angles, say A and B, is expressed as, $\cos \left( A+B \right)=\cos A\cos B-\sin A\sin B$ Thus, for angles x and y, $\cos \left( x+y \right)=\cos x\cos y-\sin x\sin y$