How directrix for other than parabola?

  • #1
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I do not understand this: An ellipse has a directrix?

A parabola, for sure. This is part of the definition and we can derive the equation of a parabola using the meaning of Directrix and the meaning of Parabola. How does a directrix work for an ellipse?
 

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  • #2
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I do not understand this: An ellipse has a directrix?

A parabola, for sure. This is part of the definition and we can derive the equation of a parabola using the meaning of Directrix and the meaning of Parabola. How does a directrix work for an ellipse?
See https://en.wikipedia.org/wiki/Ellipse#Directrix
There are several definitions for an ellipse, one of which uses a directrix.
 

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