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Thread: Parabola question

  1. June 8th 2010, 10:58 PM #1
    xsavethesporksx
    xsavethesporksx is offline
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    Parabola question

    Given
    x^2-4x=y+4
    Find the vertex, focus and directrix

    So i know that i need to complete the square
    x^2-4x+4=y+4+4

    (x-2)^2=(y+8)

    Vertex is at (2,-8)

    and im having a bit of trouble on the focus and directrix....

    Thanks
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  2. June 8th 2010, 11:15 PM #2
    xsavethesporksx
    xsavethesporksx is offline
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    okay i figured it out...

    (x-2)^2=a(y+8)
    Since (y+8) doesn't factor, then
    a=1
    general form is x^2=4ay
    set 4a=1 => a=1/4

    directrix @ y=-8.25
    focus @ (2,-7.75)

    gotta love when posting on here rings a bell.
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