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Math Help - Slope of tangent #3

  1. #1
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    Slope of tangent #3

    find the points on the graph of y=3x/(x-4), where the slope of the tangent is -12/25

    my work: (but I can't seem to find x)

    i took derivative of f(x)
    y'=[3(x-4)-3x(1)]/[x-4]^2
    y'=-4/(x-4)^2

    i let y' = -12/25
    and tried to solve for x.. no luck -help?
    Last edited by mr fantastic; August 12th 2009 at 04:17 PM. Reason: Changed post title
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  2. #2
    MHF Contributor red_dog's Avatar
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    y'=-\frac{12}{(x-4)^2}

    -\frac{12}{(x-4)^2}=-\frac{12}{25}\Rightarrow (x-4)^2=25\Rightarrow x-4=\pm 5\Rightarrow x_1=9, \ x_2=-1
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