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Tri-quadratic equation setup?

  1. Jun 19, 2014 #1
    Hey,

    So I am trying to do a tri-quadratic curve fit (linear regression) in excel. I have successfully completed a bi-quadratic, and it is of the form:
    x+x^2+y+y^2+x*y+b (b is calculated by LINEST in excel)

    My attempt at a tri-linear was:
    x^2+x*y+x*z+y^2+y*z+z^2+b (b is calculated by LINEST in excel)

    This equation was fairly accurate for my first few data points in the 4-curve family of curves I am attempting to curve fit, but after that the accuracy plummets.

    I am assuming I am on the right track with this, as I attained some initial accuracy, but this equation isn't 100% because I can see it failing. Any/all help appreciated.

    Note: I attempted to use the equation on http://www.rmi.ge/~kade/LecturesT.Kadeishvili/MathEconomics/Term3/Week3QuadraticLEC.pdf
    but it gave an extremely erroneous solution.
     
  2. jcsd
  3. Jun 20, 2014 #2

    mathman

    User Avatar
    Science Advisor
    Gold Member

    Shouldn't the fitting curve be:
    ax^2 + by^2 + cz^2 + etc.?
     
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