
trig transformation
Hi. My trig is pretty rusty. Could somebody remind me which techniques are used to get this transformation, and walk me through it?
$\displaystyle sin\,2\vartheta\,cos\,2\vartheta\,=\,\frac{1}{2}si n\,4\vartheta$
(You can replace theta with another letter if that is more convenient.)

Re: trig transformation
The trig identity that you need is
$\displaystyle \sin 2A = 2 \sin A\cos A.$
That can be derived from the identity
$\displaystyle \sin (A+B) = \sin A\cos B+\cos A\sin B$
by letting $\displaystyle B=A.$