Question

Which of the following are identities? Check all that apply.
A) sin(x-pi)=-sinx
B)cos(x+y) +cos(x-y)=2cosxcosy
C) cos(x+y)+cos(x-y)=cos^2 x - sin^2 y
D) sin(x+y)-sin(x-y)=2cosxsiny

Answer 1

just check some points
if it is an identity then left must equal to right
non identity fail for this test

Answer 2

check some values for x and y