## Precalculus (6th Edition) Blitzer

Identity for $\sin \left( C+D \right)$ is expressed as $\sin C\cos D+\cos C\sin D$.
From the sum formula of sines, the sine of the sum of two angles, say A and B, is expressed as, $\sin \left( A+B \right)=\sin A\cos B+\cos A\sin B$ Thus, for angles C and D, $\sin \left( C+D \right)=\sin C\cos D+\cos C\sin D$