$\cos (x+y)$

Step 1 ：Given the expression: \( \cos (x+y) \)

Step 2 ：Using the trigonometric identity: \( \cos (x+y) = \cos x \cos y - \sin x \sin y \)

Step 3 ：\( \boxed{\cos (x+y) = \cos x \cos y - \sin x \sin y} \)