# Math Help - trig transformation

1. ## 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?

$sin\,2\vartheta\,cos\,2\vartheta\,=\,\frac{1}{2}si n\,4\vartheta$

(You can replace theta with another letter if that is more convenient.)

2. ## Re: trig transformation

The trig identity that you need is

$\sin 2A = 2 \sin A\cos A.$

That can be derived from the identity

$\sin (A+B) = \sin A\cos B+\cos A\sin B$

by letting $B=A.$