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Why is it wrong to conclude a isn't the vertice (conics)

  1. Jul 29, 2011 #1
    1. The problem statement, all variables and given/known data

    Given the ellipse is centered at the origin and the information, find an equation of the ellipse.

    1) An ellipse with focus ([tex]\sqrt{5}[/tex],0), directrix; x = [tex]\frac{9}{\sqrt{5}}[/tex]

    2) An ellipse with an eccentricity of 4/5




    3. The attempt at a solution

    I have to leave right now so I can't post a usuall good attept, but basically for #1) you cannot match the directrix with the a/e, but for #2) you are supposed to take the focus = 4
     
  2. jcsd
  3. Jul 30, 2011 #2

    eumyang

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    Hate to be picky, but a focus is a point, not a number. If
    e = c/a,
    where c is the distance from the center to a focus, and a is the distance from the center to a vertex (not vertice!), then you should know what a is. All it remains is for you to find b, the distance from the center to an endpoint of the minor axis, and you're done.
     
  4. Jul 30, 2011 #3

    HallsofIvy

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    Yes, flyingpig knows that. He intended [itex]\left(\sqrt{5}, 0\right)[/itex] but, for some reason had only the squareroot in LaTeX and on a separate line.

    Good!
     
  5. Jul 30, 2011 #4

    eumyang

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    Actually, I was referring to #2, where he writes:
     
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