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Given $(a,b)$, $(c,d)$, and $(e,f)$ (assume non-collinear and $a\neq c$, $c\neq e$, and $a\neq e$), is there a generic way to find a parabolic function between the three?

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up vote 2 down vote accepted

Yes. Write $y = Ax^2 + Bx + C$. Substitute in the three points; if the values $a$, $c$ and $e$ are distinct, you get a nondegenerate system of three linear equations in three unknowns. Solve for $A$, $B$ and $C$ and you are there.

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The assumption here, of course, is that the parabola sought has an axis parallel to a coordinate axis. A parabola in general position, however, is only completely determined by four points. – J. M. Feb 1 '12 at 1:56
Correct. He seeks a quadratic. This gives one, but it has a graph so that $y$ is a function of $x$. – ncmathsadist Feb 1 '12 at 2:04
Otherwise the problem is underdetermined. – ncmathsadist Feb 1 '12 at 2:05

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