Question

Which of the following are identities? More than one may apply

A) sin (x+y) - sin (x-y) = 2cos x sin y
B) cos (x+y) + cos (x-y) = cos^2 x- sin^2 y
C) cos (x+y) + cos (x-y) = 2 cos x cos y
D) sin ( x- pi) = -sin x

Answer 1

B is not an identity.

Answer 2

The rest are?

Answer 3

http://learnix.net/wordpress/wp-content/uploads/Trig-Cheat-Sheet-1.4.pdf You will find this extremely helpful. I made it when i was in trig.