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: Second derivatives using delta notation

  1. Feb 14, 2007 #1
    1. The problem statement, all variables and given/known data

    Can some one explain why the second derivative of f(x) is (f(x+h)-2f(x)+f(x-h))/h^2? If you take the intervals to be x = 0 to x = 1 and you divide up the segment into little “hs” so that each x=h, 2h, 3h, nh, and so on?

    I see that first derivatives can be approximated using (f(x+h)-f(x-h))/2h, but if you were to try to measure the change in (f(x+h)-f(x-h))/2h, wouldn't you get (f(x+2h)-f(x)-f(x)-f(x-2h))/2h^2?

    Thanks.
     
  2. jcsd
  3. Feb 14, 2007 #2

    AlephZero

    User Avatar
    Science Advisor
    Homework Helper

    No, you would get (f(x+2h)-f(x)-f(x)-f(x-2h))/4h^2

    And if you substitute g = 2h you get the standard formula.

    Another way to derive it is to use f''(x) is approximately (f'(x+h/2)-f'(x-h/2))/h and f'(x+h/2) is approximately (f(x+h)-f(x))/h
     
  4. Feb 14, 2007 #3
    Oh I see it now. Thanks!
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook