Simplify the expression algebraically.
$\cos ( 6 x + 9 y ) + \cos ( 6 x - 9 y )$
A) $\cos 6 x$
B) $\cos 6 x \cos 9 y$
C) $2 \cos 6 x \cos 9 y$
D) $2 \cos 6 x$
E) $2 \cos x \cos y$

Question

Simplify the expression algebraically. 
 $\cos ( 6 x + 9 y ) + \cos ( 6 x - 9 y )$  
A) $\cos 6 x$
B) $\cos 6 x \cos 9 y$
C) $2 \cos 6 x \cos 9 y$
D) $2 \cos 6 x$
E) $2 \cos x \cos y$

Quizplus · Accepted Answer

This expression can be simplified using the trigonometric identity for the sum and difference of two angles: $\cos(A + B) + \cos(A - B) = 2 \cos A \cos B$. Here, $A = 6x$ and $B = 9y$, so the simplified form is $2 \cos 6x \cos 9y$.

Simplify the Expression Algebraically cos⁡(6x+9y)+cos⁡(6x−9y)\cos ( 6 x + 9 y ) + \cos ( 6 x - 9 y )cos(6x+9y)+cos(6x−9y)

Simplify the Expression Algebraically $\cos ( 6 x + 9 y ) + \cos ( 6 x - 9 y )$