#### Answer

Identity for $\cos \left( x+y \right)$ is expressed as $\cos x\cos y-\sin x\sin y$.

#### Work Step by Step

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,
$\cos \left( x+y \right)=\cos x\cos y-\sin x\sin y$