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Quadritic curves, how can foci = vertice?

  1. Aug 2, 2011 #1
    1. The problem statement, all variables and given/known data


    [PLAIN]http://img688.imageshack.us/img688/5336/unledaty.jpg [Broken]


    3. The attempt at a solution

    I underlined a = 3, which doesn't make sense seeing the foci is at (plus/minus3, 3)

    How can this be?? Wouldn't that make a straight line?
     
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Aug 3, 2011 #2

    eumyang

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    Homework Helper

    You're confusing distances and points.

    a is the distance between the center and a vertex. The vertices are where the blue curve intersects the focal axis. This distance a = 3.

    I use the letter c to indicate the distance between the center and a focus. You can see that while one of the foci is (3, 3), the distance c between the center (which is (0, 0)) and this focus is
    [itex]3\sqrt{2}[/itex]
    , not 3.
     
  4. Aug 3, 2011 #3
    Oh right okay thanks lol
     
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