1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: 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
    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;
    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


    User Avatar
    Homework Helper
    Gold Member
    2017 Award

    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
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook