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Math Help - Trig indentity problem, I think

  1. #1
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    Trig indentity problem, I think

    3(1+\cos\frac{\pi}{9}+\cos\frac{2\pi}{9})\sin120 \pi t +3(\sin\frac{\pi}{9}+\sin\frac{2\pi}{9})\cos120 \pi t

    = (8.638\cos\frac{\pi}{9})\sin120 \pi t+(8.638\sin\frac{\pi}{9})\cos120 \pi t

    Can anyone give me a hint about how to get from the first line to the second? I know the problem resolves to

    8.638\sin(120 \pi t + \frac{\pi}{9})
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  2. #2
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    Re: Trig indentity problem, I think

    In the first bracket use a trig identity to add 1+\cos(2\pi/9), you should get 2\cos^{2}(\pi/9).
    Remove \cos(\pi/9) as a common factor.

    In the second bracket, \sin(2\pi/9)=2\sin(\pi/9)\cos(\pi/9).
    Remove \sin(\pi/9) as a common factor.

    In both bracketed expressions, put in the decimal value for \cos(\pi/9).
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