The trigonometric functions I must know:
(A) $\sin(-x)=-\sin x$
(B) $\cos(-x)=\cos x$
(C) $\cos(x+y)=\cos x\cos y-\sin y\sin x$
(D) $\sin(x+y)=\sin x\cos y+\cos x\sin y$
$\sin^2x+\cos^2x=1$ (Use (C) and $\cos0=1$)
Can anyone help me just understand what the first one is asking. It's giving me a property and I'm supposed to derive the identities. But I don't even know where to begin. Any help? I will post the other problems once I figure this one out.
Best Answer
Hint: Note that by (A) and (B) $$\cos^2(x)+\sin^2(x)=\cos(x)\cos(-x)-\sin(-x)\sin(x)$$ and apply (C).