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Homework Help: Trying to find the equation of a cubic

  1. Jan 17, 2009 #1
    1. Find an equation that best fits the data?
    Data
    2 16.40
    3 15.70
    4 15.30
    5 15.20
    6 15.21
    7 15.40
    8 15.80
    9 16.30
    10 16.80
    11 17.50
    12 18.18
    13 18.70
    14 19.36
    15 19.88
    16 20.40
    17 20.85
    18 21.22
    19 21.60
    20 21.65

    I have narrowed it down that it is most likely a cubic, because it seems to make sense.


    I tried to use a matrix to solve this equation but found that it gave me a quartic, that did not match the data at all any help would be appreciated, is there a formula on how to solve cubics?
     
  2. jcsd
  3. Jan 17, 2009 #2

    HallsofIvy

    User Avatar
    Science Advisor

    If you mean "find the least squares best fit cubic", you would want to write, in general, [itex]y= ax^3+ bx^2+ cx+ d[/itex] and then find a, b, c, d to minimize [itex]\sum (y_i- (ax_i^3+ bx_i^2+ cx_i+ d[/itex]
     
  4. Jan 17, 2009 #3
    Have you tried simply using Finite differences?
     
  5. Jan 17, 2009 #4
    not really sure how i would use it, but ill try to
     
    Last edited: Jan 17, 2009
  6. Jan 17, 2009 #5
    I did a regression by least squares and it seems to almost perfectly fit a cubic.
    a = -0.00407
    b= 0.15356
    c=-1.2757
    d=18.27288
    (approximate values)
    where it is in the form y = ax^3 + bx^2 + cx + d
    normally you would use a computer package like MatLab or a programmable calculator to do these. I don't recommend doing this by hand, it's very long.
     
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