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Solving for x-cubic function

  1. Oct 11, 2011 #1
    1. The problem statement, all variables and given/known data
    x3-8x+5


    2. Relevant equations



    3. The attempt at a solution
    I am stuck here. I tried factoring.
     
  2. jcsd
  3. Oct 11, 2011 #2

    SammyS

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    There's no question here.

    What are you trying to do / get to ??
     
  4. Oct 11, 2011 #3
    Sorry... solve for x
     
  5. Oct 11, 2011 #4

    Mark44

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    You can't solve for x - you don't have an equation.
     
  6. Oct 11, 2011 #5
    x3-8x+5 is the equation
     
  7. Oct 11, 2011 #6

    Mark44

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    This is NOT an equation. An equation ALWAYS has an = in it.
     
  8. Oct 11, 2011 #7
    Sorry...x3-8x+5=0
     
  9. Oct 11, 2011 #8

    Mark44

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    OK, finally an equation.

    Your polynomial doesn't have any nice (i.e., rational) roots, so short of using a very complicated formula from about the 16th century, it looks to me like you need to get the root or roots by graphing the function or using some approximation technique.

    What is the complete problem description?
     
    Last edited: Oct 11, 2011
  10. Oct 11, 2011 #9
    Sorry about my terrible earlier posts...

    The problem asks me to solve x3-8x+5=0 algebraically

    heres what i have gotten to so far. Not sure if this will help me:

    i made the original equation into x(x-√8)(x+√8)+5=0
     
    Last edited: Oct 11, 2011
  11. Oct 11, 2011 #10

    gb7nash

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    Did you copy the problem correctly? Are you sure it's not x2-8x+5=0? Otherwise, your problem has no answer (other than using the cubic formula).
     
  12. Oct 11, 2011 #11

    Mark44

    Staff: Mentor

    Are you absolutely sure this is the equation you're supposed to solve?
    x3-8x+5=0

    Has your teacher shown you a technique for solving cubic equations? If not, you can't be expected to solve this one.
     
  13. Oct 11, 2011 #12

    SammyS

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    x3-8x+5=-2

    is nicer.
     
  14. Oct 11, 2011 #13

    symbolipoint

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    supposed to be, x3-8x+5=0.

    What course are you in? What methods and techniques have you studied so far? Do you know how many and what kind of roots this equation should have?
     
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