Find the Expression as the Sine or Cosine of an Angle

Multiple Choice

Find the expression as the sine or cosine of an angle. $$\cos 9 x \cos 8 y + \sin 9 x \sin 8 y$$

A) $\cos ( 9 x + 8 y )$
B) $\cos ( 8 x - 9 y )$
C) $\cos ( 9 x - 8 y )$
D) $\sin ( 9 x - 8 y )$
E) $\sin ( 8 x - 9 y )$