Results 1 to 2 of 2

Math Help - Need help solving a cubic function for a, b, c, and d

  1. #1
    Newbie
    Joined
    Aug 2008
    Posts
    3

    Need help solving a cubic function for a, b, c, and d

    I need to solve for a, b, c, and d for a cubic equation with points (2,24), (-2,0), (0,0), and (3,0). I plugged them into the equation getting these equations:
    8a + 4b + 2c + d = 24
    -8a + 4b + -2c + d = 0
    27a + 9b + 3c = 0
    d = 0

    I put the numbers into a matrix and tried to solve with Gaussian elimination but I couldn't get rid of the 27 without a huge mess. I don't know any other way to do it. Please help.
    Follow Math Help Forum on Facebook and Google+

  2. #2
    Flow Master
    mr fantastic's Avatar
    Joined
    Dec 2007
    From
    Zeitgeist
    Posts
    16,948
    Thanks
    5
    Quote Originally Posted by mono1357 View Post
    I need to solve for a, b, c, and d for a cubic equation with points (2,24), (-2,0), (0,0), and (3,0). I plugged them into the equation getting these equations:
    8a + 4b + 2c + d = 24
    -8a + 4b + -2c + d = 0
    27a + 9b + 3c = 0
    d = 0

    I put the numbers into a matrix and tried to solve with Gaussian elimination but I couldn't get rid of the 27 without a huge mess. I don't know any other way to do it. Please help.
    Have you done the obvious things? Substitute d = 0, divide equations through by common factors ..... You get:

    4a + 2b + c = 12 .... (1)

    -4a + 2b - c = 0 .... (2)

    9a + 3b + c = 0 .... (3)

    (1) + (2) => b = 3.

    Now substitute b = 3 and solve for a and c .....
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. solving cubic congruences
    Posted in the Number Theory Forum
    Replies: 2
    Last Post: September 27th 2010, 07:18 AM
  2. Solving a cubic equation.
    Posted in the Pre-Calculus Forum
    Replies: 4
    Last Post: September 23rd 2010, 12:28 PM
  3. Solving a cubic equation.
    Posted in the Pre-Calculus Forum
    Replies: 4
    Last Post: July 6th 2010, 02:55 PM
  4. Replies: 3
    Last Post: January 10th 2010, 12:56 PM
  5. Cubic function solving
    Posted in the Algebra Forum
    Replies: 4
    Last Post: February 7th 2009, 07:58 AM

Search Tags


/mathhelpforum @mathhelpforum