TITLE: Simple Trig Equation - Blank Mind
QUESTION [6 upvotes]: I'm trying to set a simple trig equation equal to zero, for use with the first derivative test, etc. I have:
$-2(\sin\theta+\sin2\theta)$
So, I need to get
$\sin\theta=-\sin2\theta$
I'm drawing a complete blank here.
If it matters, but it shouldn't the original equation I've derived from is:
$f(\theta)=2\cos\theta+\cos2\theta$
I guess what I'm really asking for is a refresher in solving trig equations, if someone doesn't mind. :)

REPLY [4 votes]: Note that $\sin(2\theta)=2\sin(\theta)\cos(\theta)$.
Wikipedia's list of trig identities should prove very helpful :)