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Maximum and Minimum Value Application question

  1. Feb 27, 2010 #1
    1. The problem statement, all variables and given/known data
    Find the value of k if the function y=x^2+kx+72 has a local minimum at x=4
  2. jcsd
  3. Feb 27, 2010 #2
    What have you tried for this problem.
  4. Feb 27, 2010 #3
    Differentiate the original function and you get y = 2x + k ,and it has a local minimun at x=4.
    Block 4 in and the output(y) should be 0 because of Fermat's theorem (http://en.wikipedia.org/wiki/Fermat's_theorem_(stationary_points))

    So you'll get k = -8 ,is that correct ?
  5. Feb 27, 2010 #4
    yes! thankyou so much :)
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