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Math Help - integrl question..

  1. #1
    MHF Contributor
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    integrl question..

    \int_{-\pi}^{\pi}\sin x \sin 2x

    i tried to solve it by parts
    but both sides change to cosine

    ??
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  2. #2
    Super Member flyingsquirrel's Avatar
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    Quote Originally Posted by transgalactic View Post
    \int_{-\pi}^{\pi}\sin x \sin 2x
    Use this identity : \sin p \sin q =\frac{\cos(p-q)-\cos(p+q)}{2}.
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  3. #3
    Math Engineering Student
    Krizalid's Avatar
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    use the double angle formula for sine.
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