# Math Help - [SOLVED] Use these equalities to derive the following trig identities.

1. ## [SOLVED] Use these equalities to derive the following trig identities.

(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!

2. $\cos (x-y) = \sin x \sin y + \cos x \cos y$

$\sin^{2} x + \cos^{2} x = \sin x \sin x + \cos x \cos x = \cos (x-x)= \cos 0 =1$