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Math Help - increasing and decreasing functions

  1. #1
    Junior Member
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    increasing and decreasing functions

    I am to determine the interval on which f(x)=12x-x^3 is decreasing. I am coming up with x(12-x^2). x plus or minus =2square root 3. Solution being (-infinity, -2 square root 3). I have no other way to determine if I am doing this right. Can someone please set me straight?
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  2. #2
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    You have to find when f '(x) is negative. You don't seem to have differentiated yet??
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