Use a sum-to-product identity to rewrite the expression.
-cos 8s - cos 5s
A) 2 cos(6.5s) cos(1.5s)
B) 2 sin(6.5s) cos(1.5s)
C) -2 cos(6.5s) sin(1.5s)
D) -2 sin(6.5s) sin(1.5s)

Question

Quizplus · Accepted Answer

The sum-to-product identity for cos A - cos B is -2 sin((A+B)/2) sin((A-B)/2). Here, A = 8s and B = 5s, so it becomes -2 sin((8s+5s)/2) sin((8s-5s)/2) = -2 sin(6.5s) sin(1.5s).

Use a Sum-To-Product Identity to Rewrite the Expression