# Trig Identities

1. Aug 24, 2005

### cscott

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

2. Aug 24, 2005

### Jameson

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.

3. Aug 24, 2005

### TD

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 $\left( {3\sin x - 2\cos x} \right)^\prime = 3\left( {\sin x} \right)^\prime - 2\left( {\cos x} \right)^\prime$

4. Aug 24, 2005

### cscott

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

5. Aug 24, 2005

### Hurkyl

Staff Emeritus
You don't need trig identities to differentiate those...