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Thread: Trigonometric Identity

  1. August 13th 2009, 05:00 AM #1
    Mike9182
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    Trigonometric Identity

    What trigonometric identity makes the following true?

    \sin(x)*\sin(y)=\frac{1}{2}[\cos(x-y)-\cos(x+y)]
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  2. August 13th 2009, 05:12 AM #2
    skeeter
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    Quote Originally Posted by Mike9182 View Post
    What trigonometric identity makes the following true?

    \sin(x)*\sin(y)=\frac{1}{2}[\cos(x-y)-\cos(x+y)]

    \frac{1}{2}[\cos(x-y)-\cos(x+y)]

    use the sum\difference identity for cosine ...

    \frac{1}{2}[\cos{x}\cos{y} + \sin{x}\sin{y} - (\cos{x}\cos{y} - \sin{x}\sin{y})]
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