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I can define a curve that passes through 3 points using a quadratic equation:

ax2 + bx + c = 0

I would like to know is it possible to define a curve that passes through 4 points using:

ax3 + bx2 + cx + d = 0

Cheers

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migrated from stackoverflow.com Feb 12 '12 at 8:10

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Yes, it is possible. In general, for any n points in a plane, you can find an (n-1)th degree (or higher) polynomial that passes through all of them. Finding these polynomial involves solving matrix equations and can sometimes get a bit messy. –  jpm Feb 10 '12 at 23:37
    
You can also use a Lagrange interpolating polynomial. –  Anne Nonimus Feb 10 '12 at 23:40
    
@jpm is correct. For an easy formula (that works in most cases), see Lagrange polynomial - Wikipedia, the free encyclopedia. –  Dennis Feb 10 '12 at 23:41

1 Answer 1

The answer was already in the comments upon migration: Use a Lagrange polynomial. The restriction "in most cases" is unnecessary; the Lagrange polynomial is completely general and yields a polynomial which interpolates the points as long as no two of them have the same $x$ coordinate; if they do, there can be no univariate function, polynomial or otherwise, that interpolates them.

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