Results 1 to 3 of 3

Math Help - parabolas

  1. #1
    Newbie
    Joined
    Mar 2008
    Posts
    5

    parabolas

    x = -1/2y squared - y - 4
    Follow Math Help Forum on Facebook and Google+

  2. #2
    Newbie
    Joined
    Mar 2008
    Posts
    5

    conic sections help please

    x = -1/2y squared - y - 4

    i need the vertex, x intercept, y intercepts and directrix...that would be marvelous.
    Follow Math Help Forum on Facebook and Google+

  3. #3
    Super Member
    earboth's Avatar
    Joined
    Jan 2006
    From
    Germany
    Posts
    5,804
    Thanks
    115
    Quote Originally Posted by brittneyv View Post
    oops i forgot about that part.
    sorry, i need the vertex, directrix, x-axis and y-axis.
    Re-write the equation into the vertex-directrix-form:

    x = -\frac12 y^2 - y - 4 ~\iff~-2x = y^2 + 2y +1-1+8 ~\iff~-2\left(x+\frac72 \right) = (y+1)^2~\iff~ 2 \cdot (-1)\cdot \left(x+\frac72 \right) = (y+1)^2

    The vertex has the coordinates V\left(-\frac72 , -1 \right)

    Therefore the directrix has the equation d: x = -\frac72 -\left(-\frac12 \right) = -3

    With your need for x-axis and y-axis I assume that you mean the x and y intercepts of the parabola:

    x-intercept: y = 0:

    2 \cdot (-1)\cdot \left(x+\frac72 \right) = (0+1)^2~\iff~ x = -4

    y-intercept: x = 0:

    2 \cdot (-1)\cdot \left(0+\frac72 \right) = (y+1)^2 The LHS is negative, the RHS is a square - but a square can't be negative and therefore there doesn't exist any y-intercept.

    Another attempt: The vertex is left of the y-axis and the parabola opens to left. Therefore the parabola and the y-axis can't have a common point. Thus: No y-intercept.
    Attached Thumbnails Attached Thumbnails parabolas-pardirctvert.gif  
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. Parabolas please help!
    Posted in the Pre-Calculus Forum
    Replies: 1
    Last Post: June 4th 2010, 10:31 AM
  2. I Need Help With Parabolas!!!
    Posted in the Pre-Calculus Forum
    Replies: 3
    Last Post: May 26th 2008, 03:42 PM
  3. Parabolas
    Posted in the Pre-Calculus Forum
    Replies: 5
    Last Post: May 5th 2008, 12:25 PM
  4. parabolas
    Posted in the Pre-Calculus Forum
    Replies: 14
    Last Post: April 1st 2008, 06:58 PM
  5. Parabolas- Need help
    Posted in the Pre-Calculus Forum
    Replies: 3
    Last Post: November 24th 2007, 03:22 PM

Search Tags


/mathhelpforum @mathhelpforum