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I have 3 random points in an ellipse. Is it possible to find the radius of the ellipse?

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    @TonyK Upps ... you are right of course.2012-05-02

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The equation of an ellipse whose major and minor axes are parallel to the Cartesian axes is:

$ \left(\frac{x-x_{0}}{a}\right)^{2}+\left(\frac{y-y_{0}}{b}\right)^{2}=1 $

As you can see, the equation contains 4 different variables. Therefore, 3 points aren't sufficient to uniquely identify the ellipse.