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I How directrix for other than parabola?

  1. Aug 28, 2016 #1

    symbolipoint

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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?
     
  2. jcsd
  3. Aug 28, 2016 #2

    Mark44

    Staff: Mentor

    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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