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Help with Quadratic Equations?

  1. Jul 2, 2004 #1
    Any idea how to figure this problem out?

    x + 1 = 9x^3 + 9x^2
  2. jcsd
  3. Jul 2, 2004 #2


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    First off, this belongs in a different section. General math, or homework help.

    It's fairily easy to solve. What have you tried?
  4. Jul 3, 2004 #3


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    In addition to NateTG's point that this is NOT a differential equation, it also is NOT a quadratic equation. It is a cubic equation that can be reduced to the standard form 9x3+ 9x2- x- 1= 0.

    There exist a general formula for cubics but it is difficult to apply. Sometimes the simplest way to solve an equation is to cross your fingers and see what happens if you plug in simple numbers for x! (Hint: try negative numbers first.)
  5. Jul 13, 2004 #4
    Hi ,
    You can solve this equation using the cardon methods which can be found in the any linear algebra book
  6. Jul 13, 2004 #5
    Hi ,
    You can solve this equation using the cardon methods which can be found in the any linear algebra book

  7. Aug 13, 2004 #6
    This one is simple. Really simple actually...

    x + 1 = 9x^3 + 9x^2
    x + 1 = 9x^2(x + 1)
    9x^2(x + 1) - (x + 1) = 0
    (x + 1)(9x^2 - 1) = 0
    x + 1 = 0 => x = -1
    9x^2 - 1 = 0 => x = +- 1/3

    x_1 = -1
    x_2 = 1/3
    x_3 = -1/3

    No need for cardano or anything
  8. Aug 16, 2004 #7


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    Or simply try x= -1 as I originally suggested to find that
    -1+1= 0= 9(-1)3+ 9(-1)2 so that x= -1 is a root. Once you know that, it is easy to see that 9x3+ 92-x-1=(x+1)(9x2-1)= (x+1)(3x-1)(3x+1).
  9. Aug 19, 2004 #8
    Does anyone know a link to the proof which shows that there exists no exact solution for equations of order 5 and above? Sorry I don't happen to know the exact terms for this.
  10. Aug 19, 2004 #9
    It is not gonna be easy. Type Galois on google. Don't read his theory, it is too difficult. Read about his life, it is fascinating. We are very proud of Galois back in France :approve:

    To Dinesh : they are also proud of Cardan back in Italy. :grumpy:
  11. Aug 19, 2004 #10

    matt grime

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    There is a thread in this forum with galois stuff in it. titled cubic formula started by atcg, it's on the first page of the current topics somewhere.
  12. Aug 19, 2004 #11


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    It's not, by the way, true that "there is no exact solution". What is true is that there exist polynomial equations with solutions that cannot be written as radicals (roots of roots of .... rational numbers).
    As pointed out, Galois theory is very deep. Essentially, group theory originated as a way or simplifying the proof!

    The idea:
    1. A polynomial equation is "solvable by radicals" if and only if the Galois group of the polynomial is a "solvable group".
    2. For any positive integer, n, there exist a polynomial of degree n whose Galois group is isomorphic to Sn, the group of permutations on n objects.
    3. If n> 4, Sn is not a solvable group.
  13. Aug 19, 2004 #12
    Galois died so young. He would have brought to humanity such a wider math landscape. :cry:
  14. Aug 19, 2004 #13
    IIRC from a 6th grade report on him didn't he die in a duel? tsk tsk. Different times, I suppose.
  15. Aug 20, 2004 #14


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    Yes, he fought a duel with a rival over some girl who wasn't worth half of him.

    There was, by the way, a suggestion that the duel was set up by the duel was "set up" by government agencies. This was during the short time right after the defeat of Napolean when the Bourbons were back on the throne. There is, in fact, no evidence for that, although Galois did spend a year in prison just before this for "threatening the life of the King".
  16. Aug 20, 2004 #15
    It is very much more likely that he did not care about the girl. He constantly said that he would give his life if that could trigger revolution. Apparently, he decided himself to organise the duel, and die. But the same day, other events (a general murdered) triggered the "commune" revolution. So after all, he died for nothing.

    The truth is : nobody knows for sure. That is what is fascinating. The thing is : Galois was so clever, why would he stupidely die in a duel ?
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