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-   -   Graph parabola (https://www.askmehelpdesk.com/showthread.php?t=330673)

  • Mar 17, 2009, 10:51 PM
    ilovemattc
    Graph parabola
    Graph the following parabola (x-2)^2=(y+1) list the vertex focus and directrix
  • Mar 18, 2009, 10:01 AM
    galactus
    Rearranging, we have:



    That means we have the form

    where h and k are the vertex coordinates.

    The directrix is the same distance from the vertex as the focus is from the vertex.
  • Mar 19, 2009, 10:17 AM
    galactus
    1 Attachment(s)
    Pardon my typo in my first post. That should be

    Note, if we expand we get

    The x coordinate of the vertex can also be easily obtained by using

    Then, sub that value back into the quadratic to find its corresponding y coordinate.

    Note, you have the form . This is a concave up parabola.

    The distance from the vertex to the focus is .

    Consequently, that distance is the same from the vertex to the directrix. You have found the vertex, so use that and find the directrix.

    It will just be a horizontal line with coordinates y=some number. Look at the graph. That'll help.

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