Use a product-to-sum formula to convert the expression to a sum or difference. Simplify where possible. $\cos \frac { 5 \pi } { 12 } \cos \frac { \pi } { 12 }$
A) 0
B) -1/4
C) -1/2
D) 1/4
E) 1/2

Question

Quizplus · Accepted Answer

Using the product-to-sum formula: $\cos A \cos B = \frac{1}{2}[\cos(A+B) + \cos(A-B)]$, we get $\frac{1}{2}[\cos(\frac{6\pi}{12}) + \cos(\frac{4\pi}{12})] = \frac{1}{2}[\cos(\frac{\pi}{2}) + \cos(\frac{\pi}{3})] = \frac{1}{2}[0 + \frac{1}{2}] = \frac{1}{4}$.

Use a Product-To-Sum Formula to Convert the Expression to a Sum