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Homework Help: Sketching ln graph

  1. Jun 11, 2008 #1
    I have to sketch the following function:
    y=x(lnx)^3

    The ln scares me a bit... I have found the derivative of the function as 3(lnx)^2, and when making that equal to zero I got an anwer of around 0.7 if i remember correctly. This I guess is my critical number??? Then the second derivative is 6/x(lnx). I really am not sure if Im doing this right, especially with the ln in there. Am I on the right track? What can I now do?
     
  2. jcsd
  3. Jun 11, 2008 #2

    Dick

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    You'd better try the first derivative again. You'll need to use both the product rule and the chain rule.
     
  4. Jun 11, 2008 #3
    You are missing one term:
    it should be
    [..]+6*log(x)/x

    yep, so you are maximum points are wrong..
    you should be getting
    1.0000
    0.0498

    ignore what the thing is.. find values when function is max, and see if f^2 is positive for x>0 ...

    I would use software like "Graph: padowan.dk
    and my calculator (plugging in different values)
     
  5. Jun 11, 2008 #4

    tiny-tim

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    Hi emma3001! :smile:

    Hint: try it for cosx instead of lnx, and then adapt.

    So try y=x(cos)^3, and y = 6/x(cosx). :smile:
     
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