# Trig Identities

cscott
How can I use trig identities to let me differentiate $\sin \left ( x + \frac{\pi}{2} \right)$ and $3 \sin x - 2 \cos x$?

## Answers and Replies

Gold Member
MHB
Well, there's an identity for $$\sin{(\alpha+\beta)}$$ but why would you want to use it? Those can be differentiated easily... I'm missing something.

Homework Helper
Well, $\sin \left ( x + \frac{\pi}{2} \right) = \cos x$ if that helps

I don't see why you'd want to change the second one though, taking the derivative is lineair so $\left( {3\sin x - 2\cos x} \right)^\prime = 3\left( {\sin x} \right)^\prime - 2\left( {\cos x} \right)^\prime$

cscott
Damn, I guess I wasn't looking at them right. Thanks.

Staff Emeritus