Circles and ellipses

The entire discussion about whether a circle is an ellipse hinges on how you define circle and ellipse.

If you take the classical definition of an ellipse, from Euclidean geometry, defined based on the sum of the distances from each point on the ellipse to the two foci, then a circle is NOT an ellipse. A circle does not have two foci.

I suppose it also depends on what the meaning of the word "is" is.

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Regards,
    David Cressey
    www.dcressey.com