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) $\sin ( 8 x - 9 y )$

Question

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) $\sin ( 8 x - 9 y )$

Quizplus · Accepted Answer

This expression is an application of the cosine addition formula, $\cos(A + B) = \cos A \cos B + \sin A \sin B$, which directly matches the given expression with $A = 9x$ and $B = -8y$, resulting in $\cos(9x - 8y)$.

Find the Expression as the Sine or Cosine of an Angle