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Homework Help: Find derivative of function given the values

  1. Sep 17, 2012 #1
    1. The problem statement, all variables and given/known data
    You are given the following values for the errror function f HxL = erf HxL.
    erf (0)= 0
    erf (1) = 0.84270079
    erf(.1)= 0.11246292
    erf(.01) = 0.01128342
    Use this information to approximate the value of erf '(1). Explain how you arrived at your answer.

    2. Relevant equations

    3. The attempt at a solution
    I basically took the average from both sides of erf(.01)
    (((erf(0)-erf(.01))/.01)+((erf(.01)-erf(.1))/(-.09)))/2 = (1.128342 + 1.124216667)/2 =1.126279335

    I ended up with 1.126279335 so i'm assuming its something close to that like 1.12? Am I right or wrong?
  2. jcsd
  3. Sep 17, 2012 #2


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    Gold Member

    Why would you use values near 0 when you are interested in erf'(1)? Did you type the last two data points correctly?
  4. Sep 17, 2012 #3


    Staff: Mentor

    What is the HxL business?
    I can't see that this will do you much good. To get an estimate of the derivative at x = 1, you need values that are reasonably close to x = 1. The two that are closest are x = .1 and x = 1.
    Your estimate is too large by at least a factor of 2.
  5. Sep 17, 2012 #4
    Sorry i meant to say at erf'(0).that was a typo on my part
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