Results 1 to 3 of 3

Math Help - Cubic Polynomial

  1. #1
    Newbie
    Joined
    Oct 2008
    Posts
    2

    Cubic Polynomial

    So basically I have been given two roots of 2+i and 3. It asks what the cubic polynomial is, and sadly I have forgotten how to do this all together. Thanks in advance.
    Follow Math Help Forum on Facebook and Google+

  2. #2
    Banned
    Joined
    Aug 2008
    Posts
    530

    Reply

    If one root is (2 + i) then other root must be (2 - i) because the complex roots always occur in conjugate-pairs.
    So, the three roots are (2 + i), (2 - i) and 3.

    so, the cubic equation is

    [x-(2 + i)][x-(2 - i)][x-3]=0

    (x-2 - i)(x-2 + i)(x-3)=0

    (x^2-2x+ix-2x+4-2i-ix+2i-i^2)(x-3)=0

    (x^2-4x+5)(x-3)=0

    x^3-3x^2-4x^2+12x+5x-15=0

    x^3-7x^2+17x-15=0
    Follow Math Help Forum on Facebook and Google+

  3. #3
    Banned
    Joined
    Aug 2008
    Posts
    530

    Graph

    Please see attached graph.
    Attached Thumbnails Attached Thumbnails Cubic Polynomial-graph7.jpg  
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. [SOLVED] cubic polynomial
    Posted in the Algebra Forum
    Replies: 2
    Last Post: January 2nd 2011, 01:20 AM
  2. Cubic Polynomial
    Posted in the Algebra Forum
    Replies: 5
    Last Post: June 23rd 2010, 11:01 AM
  3. A cubic Polynomial
    Posted in the Calculus Forum
    Replies: 1
    Last Post: November 4th 2009, 01:22 PM
  4. cubic polynomial...
    Posted in the Calculus Forum
    Replies: 3
    Last Post: September 24th 2009, 08:39 AM
  5. cubic polynomial
    Posted in the Algebra Forum
    Replies: 1
    Last Post: March 22nd 2008, 09:53 AM

Search Tags


/mathhelpforum @mathhelpforum