(A) sin(-x) = -sinx

(B) cos(-x) = cosx

(C) cos(x+y) = cosx cosy + sinx siny

(D) sin(x+y) = sinx cosy + cosx siny

I'm stuck on the very first question:

(a) sin²x + cos²x = 1 (use (C) and cos0 = 1)

the answers also say to use (A) and (B).. but I dunno how that would apply. It also said to use x = y. How am I supposed to use cos0 though?

thanks in advance for the help!