Find the expression as the sine or cosine of an angle.
$\cos 9 x \cos 5 y + \sin 9 x \sin 5 y$
A) $\sin ( 5 x - 9 y )$
B) $\sin ( 9 x - 5 y )$
C) $\cos ( 5 x - 9 y )$
D) $\cos ( 9 x - 5 y )$
E) $\cos ( 9 x + 5 y )$

Question

Find the expression as the sine or cosine of an angle. 
 $\cos 9 x \cos 5 y + \sin 9 x \sin 5 y$  
A) $\sin ( 5 x - 9 y )$
B) $\sin ( 9 x - 5 y )$
C) $\cos ( 5 x - 9 y )$
D) $\cos ( 9 x - 5 y )$
E) $\cos ( 9 x + 5 y )$

Quizplus · Accepted Answer

Using the angle addition formula for cosine, we have:$\cos 9 x \cos 5 y + \sin 9 x \sin 5 y = \cos (9x - 5y)$

Find the Expression as the Sine or Cosine of an Angle