Use a product-to-sum identity to rewrite the expression.
-cos 43° cos 20°
A) 0.5(sin (23°) + sin(63°))
B) 0.5(sin(63°) - sin (23°))
C) 0.5(cos (23°) + cos(63°))
D) 0.5(cos (23°) - cos(63°))

Question

Quizplus · Accepted Answer

The product-to-sum identity for cosine is $\cos A \cos B = \frac{1}{2}[\cos(A - B) + \cos(A + B)]$. Applying this to $\cos 43° \cos 20°$ gives $\frac{1}{2}[\cos(43° - 20°) + \cos(43° + 20°)] = \frac{1}{2}[\cos(23°) + \cos(63°)]$.

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