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Math Help - Very quick trig identity check

  1. #1
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    Very quick trig identity check

    trying to take the integral of cos^2(3x)

    but since I can take that integral, I need to change it up. just need to know if I'm doing this correctly.. I'm getting..

    1/2 + 1/2cos(6x)

    ?
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  2. #2
    MHF Contributor Calculus26's Avatar
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    No

    Let u = 3x du = 3dx


    1/3integral(cos^2(u)) = 1/3 (u/2 +1/4sin(u))= x/2 +1/12[sin(3x)]
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  3. #3
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    \frac{1}{2}\int[1+cos6x]
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  4. #4
    MHF Contributor Calculus26's Avatar
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    Of course my answer should have been 1/3 (u/2 +1/4 sin(2u) )=

    x/2 +1/12sin(6x)
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