Question

Which of the following are identities? Check all that apply.

A. sin(x+y)+ sin(x - y) = 2cos x siny
B. cos(x + y) + cos(x- y) = 2cos x cosy
C. tan(x - 2) = tan x
D. COS(x + y) - cos(x - y) = 2 cos xcosy

Answer 1

B. cos(x+y) + cos(x-y)= 2cosx.cosy

Explanation:

cos(x+y) + cos(x-y)

= cosx.cosy - sinx.siny + cosx.cosy + sinx.siny

=cosx.cosy + cosx.cosy

=2cosx.cosy