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[–] sillyquiet ago  (edited ago)

not all ellipses are circles. Circles, however, are a special case of ellipse in which the both foci coincide, which means some ellipses are in fact circles.

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[–] sqeak ago  (edited ago)

Please demonstrate, I don't understand, all I can find is this.

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[–] sillyquiet ago 

As usual, Wikipedia illustrates better than I can. See here: https://en.m.wikipedia.org/wiki/Ellipse, under "elements of an ellipse". The last paragraph mentions that when the eccentricity, that is the ratio of the distance between the foci and the length of the major axis, is zero the ellipse is a circle. An analogy would be circle:ellipse as coupe:automobile. Not all automobiles are coupes, but all coupes are automobiles.