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Math Help - trig transformation

  1. #1
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    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.)
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  2. #2
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    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.
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