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Concavity of Parametric Equations

  1. Mar 18, 2014 #1

    sheldonrocks97

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    Gold Member

    1. The problem statement, all variables and given/known data

    Find dy/dx and d^y/dx^2

    x=e^t; y=te^(-t)

    For which values of t are concave upward? (write your answer in interval notation).

    2. Relevant equations



    3. The attempt at a solution

    I used the formula to find d^2y/dx^2.

    d^2y/dx^2= e^(-3t)*(2t-3)

    Set it to zero:

    e^(3t)*(2t-3)>0

    I solved it and got t>3/2, but the computer told me it was wrong. What am I doing wrong here?
     
    Last edited: Mar 18, 2014
  2. jcsd
  3. Mar 18, 2014 #2

    LCKurtz

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    Where did the exponential come from? You only have cosines in the problem. After you fix that, please show your work so we can follow it.
     
  4. Mar 18, 2014 #3

    Mark44

    Staff: Mentor

    The OP edited his post, so the parametric equations now have exponential form
    Yes. Show your work for dx/dt and dy/dt and so on.
     
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