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Math Help - Derivation problem

  1. #1
    Junior Member
    Joined
    Sep 2009
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    32

    Derivation problem

    y = (cos7x)^x

    lny = x(lncos7x)

    y/y' = (x ) 1/(cos7x) (-sin7x) + lncos7x

    y/y' = -xtan7x + lncos7x

    y' = ((cos7x)^x) (-xtan7x + lncos7x)

    Is this wrong? Why?
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  2. #2
    Super Member
    Joined
    Aug 2009
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    Israel
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    976
    (cos(nx))' = -n \cdot sin(nx)

    You forgot the n the answer is y' = (cos^x(7x)) \cdot (log(cos(7x)) - 7xtan(7x))
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