Quadratic Formula

Discussion in 'Math' started by Thinker, Feb 1, 2007.

  1. Thinker

    Thread Starter Active Member

    Jan 9, 2007
    61
    0
    Okay.

    I have to solve a algebraic expression using a quadratic formula.

    This is it:

    X2 - 2X - 6 = 0

    I used the quadratic formula on your site here to help me. So...

    a=1, b=-2, c=-6


    X= +3 ± |4 - 4 * 1 - (-6)
    -------------------------
    2 * 1

    = 3 ± |0
    ----------
    2

    = 3 ± 0
    --------
    2

    .........and I got 1.5

    But when i put it to the equation X2 - 2X - 6 = 0 I get 0.75, it should be 0 right?

    Does anybody know where i possible went wrong?
     
  2. Papabravo

    Expert

    Feb 24, 2006
    9,898
    1,722
    If b = -2 then minus b must be ceteris paribas 2. N'est pas?
     
  3. Dave

    Retired Moderator

    Nov 17, 2003
    6,960
    143
    Hi Thinker,

    You appear to be a little off course with this one. Papabravo is correct: --2 = +2. Additionally, what you have in the square root is wrong:

    If b = -2, then b^2 = +4

    And if a = 1 and c = -6, then 4ac = 4*1*-6 = -24

    So your expression in the square root is (4 - -24) = +28

    If you follow the quadratic equation exactly you should get the answer to be x = +3.64575 or -1.64575

    Have another go and post up if you have any problems.

    Dave
     
  4. Thinker

    Thread Starter Active Member

    Jan 9, 2007
    61
    0
    Thanks Papabravo and Dave

    I'll test it out and get back to you, I'm a bit busy now.
     
  5. Thinker

    Thread Starter Active Member

    Jan 9, 2007
    61
    0
    Okay i worked everything out again and this is what i got:

    (3.64575)2 - (2 * 3.64575) - 6 = 0
    13.29150 - 7.2915 - 6 = 0

    That is for the 3.64575 answer. I don't know what i was thinking when i put -3 in the equation instead of -2! :confused:

    Thanks Dave and for the help. :)
     
  6. Dave

    Retired Moderator

    Nov 17, 2003
    6,960
    143
    Don't forget that x = -1.64575 is also a solution to the equation. This arises due to the fact that there is a +/- in the quadratic formula and provided 4ac =/ 0 then there will be two solutions.

    Dave
     
  7. Thinker

    Thread Starter Active Member

    Jan 9, 2007
    61
    0
    Okay Dave, thanks.

    (sorry i know this reply is a bit late)
     
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