Results 1 to 2 of 2

Math Help - foci on parabola

  1. #1
    Member
    Joined
    Nov 2005
    Posts
    172

    foci on parabola

    The parabola  y = x^2+3x has its focus at the point (b, c) where
    b=
    c=
    i tried using the stardard equation  y^2 = 4px , but got lost:
    alright i did this by completing the square:
     x^2+3x = y
     x^2+3x+\frac{9}{4} = y + \frac{9}{4}
     (x+\frac{3}{2})^2 = y+\frac{9}{4}
    so b = \frac{-3}{2}, need help finding c
    Last edited by viet; July 16th 2007 at 03:39 PM.
    Follow Math Help Forum on Facebook and Google+

  2. #2
    Eater of Worlds
    galactus's Avatar
    Joined
    Jul 2006
    From
    Chaneysville, PA
    Posts
    3,001
    Thanks
    1
    The focus is a distance p from the vertex. p=\frac{1}{4a}

    Where a=1. So p=1/4. Since the vertex is at (-3/2, -9/4). The parabola opens upward, so the focus will be above the vertex. -9/4+1/4=-2

    F(-3/2, -2)
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. Foci
    Posted in the Pre-Calculus Forum
    Replies: 3
    Last Post: April 24th 2011, 05:32 AM
  2. Parabola and Foci Problems
    Posted in the Algebra Forum
    Replies: 1
    Last Post: February 20th 2011, 09:56 PM
  3. The foci of an ellipse - a bit unusual
    Posted in the Geometry Forum
    Replies: 10
    Last Post: August 19th 2009, 12:57 AM
  4. Need help finding foci of an ellipse
    Posted in the Pre-Calculus Forum
    Replies: 2
    Last Post: December 1st 2008, 12:15 PM
  5. help finding foci
    Posted in the Pre-Calculus Forum
    Replies: 4
    Last Post: June 26th 2008, 10:26 AM

Search Tags


/mathhelpforum @mathhelpforum