#### 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 difference formula of cosines, the cosine of the difference between 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,
$\cos \left( x-y \right)=\cos x\cos y+\sin x\sin y$