Question

I really need some help please!

cos(a - B) + cos(a + B) = _____
A. 0
B. 2cosacosB
C. 2sinasinB
D. 2(cosacosB + sinasinB)

Answer 1

The answer is B.

I tried putting 9 for a and 7 for B. cos(9-7)+cos(9+7) = 2cos(9)cos(7)

Answer 2

`\bf \textit{Product to Sum Identities} \\\\ cos(\alpha)cos(\beta)=\cfrac{1}{2}\left[\cfrac{}{}cos(\alpha-\beta)\quad +\quad cos(\alpha+\beta)\right] \\\\[-0.35em] \rule{34em}{0.25pt}\\\\ cos(a)cos(B)=\cfrac{1}{2}\left[\cfrac{}{}cos(a-B)+cos(a-B) \right] \\\\\\ 2cos(a)cos(B)=cos(a-B)+cos(a-B)`