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Math Help - derivative of trig function

  1. #1
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    derivative of trig function

    what is the derivative of cos^2x ?

    i know cosx = -sinx

    and i dont think cos^2x would equal -sin^2x because i would probably more complicated then that... lol

    am i right or am i wrong... i need some guidance please. thank you.
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  2. #2
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    Write cos^2 x as (cos x)^2 and then use the chain rule.
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  3. #3
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    okay so this is how i did it
    cosx^2

    y= u^2
    dy/du = 2u
    u= cosx
    dy/dx = -sinx
    x=

    dy/dx = dy/du X du/dx
    = 2u X (-sinx)
    = -2cosxsinx
    = -sin2x ? <cause i use that identity because it is sin2x = 2sinxcosx...
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  4. #4
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    is that right?
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  5. #5
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    Perfect. Well done. Remember that little trick for next time...and there will be a next time!
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  6. #6
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    yeah forsure thanks, i would have never thought to do that myself. =)
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