# trig transformation

• Feb 17th 2013, 09:32 PM
infraRed
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.)
• Feb 18th 2013, 01:54 AM
BobP
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 $B=A.$