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Thread: trig identity

  1. June 17th 2012, 07:49 AM #1
    srirahulan
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    Arrow trig identity

    If sin\theta+sin\phi=a,cos\theta+cos\phi=b prove that \frac{sin\frac{(\theta+\phi)}{2}}{a}= \frac{\frac{(cos\theta+\phi)}{2}}{b}= \frac{2cos(\frac{\theta-\phi)}{2}}{a^2+b^2}
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  2. June 17th 2012, 08:12 AM #2
    veileen
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    Re: trig identity

    Re-write a and b using these:

    \sin \theta+\sin \phi=2\sin \frac{\theta+\phi}{2}\cos \frac{\theta-\phi}{2}
    \cos \theta+\cos \phi=2\cos \frac{\theta+\phi}{2}\cos \frac{\theta-\phi}{2}

    Then calculate each fraction.
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