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Maxima and minima of function

  1. Oct 22, 2012 #1
    1. The problem statement, all variables and given/known data
    Consider the following function of the variable r, r>/=0
    y(r)=(r^(2)-5.50r)exp(-r)
    Find the value of the first derivative dy/dr at r=5.50

    2. Relevant equations
    How do I solve this? I know its a simple derivative equation, but I can't seem to get it. I tried finding the derivative and then plugging in r=5.50 but MasteringPhysics says it incorrect.
    Also, does exp(-r) mean to the -r power? I've never seen it written like that.


    3. The attempt at a solution
    Assuming exp(-r) means to the negative r power...
    I tried using the chain rule;
    y(r)=(r^(2)-5.50r)exp(-r)
    dy/dr=-r(r^(2)-5.50r)^(-r-1)*(2r-5.50)
    plugging in r=5.50, I get a 0, making the solution 0, which is obviously wrong...
    I think its my poor calc skills, any help would be great, thanks!
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Oct 22, 2012 #2

    TSny

    User Avatar
    Homework Helper
    Gold Member

    exp(-r) is a representation of ##e^{-r}##.

    So, your function is ##y(r) = (r^2-5.50r)e^{-r}##
     
  4. Oct 22, 2012 #3
    Thanks, would the derivative be e^-r(2r^3-16.5r^2+30.35r)? Thats what I got, and after subbing in r=5.5 again, I still get the answer wrong...
     
  5. Oct 22, 2012 #4
    What's the derivative of ex with respect to x?

    What's the derivative of e-x with respect to x?

    Do you know how to differentiate by parts?
     
  6. Oct 22, 2012 #5
    Oh ok, Ill have to reference my calc notes since im pretty rusty. but I think I should get it now. Thanks for the help
     
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