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Thread: solve for x^2 [resolved]

  1. Feb 7th, 2004, 01:17 PM #1
    dolor
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    solve for x^2 [resolved]

    I wanna know what x^2 is equal to if
    (x+9)^(1/3) - (x-9)^(1/3) = 3

    I've got this, but I'm not sure ..someone tell me if I'm using magic anywhere.

    [(x + 9)^(1/3)]^3 = [3 + (x - 9)^(1/3)]^3 //cube both sides
    x + 9 = 27 + x - 9
    (x + 9)^2 = (27 + x - 9)^2 //square both sides
    x^2 + 18x + 81 = x^2 + 36x + 324 //the x^2 cancels out

    -243 = 18x
    x = -243/18 //solve for x

    so if you square both sides again...

    x^2 = (-243/18)^2
    Last edited by dolor; Feb 9th, 2004 at 07:08 PM.
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  2. Feb 7th, 2004, 07:26 PM #2
    Acidic
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    well, my calculator seems to dissagree, I'm trying to work out why though.

    edit:
    OK, I'm pretty sure that 8.9447 is the right answer (though I don't know how to work that out without a calculator).
    I'mpretty sure it's the cubing everything that seems to upset things, but I have no clue how else I could do this.
    Last edited by Acidic; Feb 7th, 2004 at 07:33 PM.
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  3. Feb 7th, 2004, 07:38 PM #3
    dolor
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    Originally posted by Acidic
    well, my calculator seems to dissagree, I'm trying to work out why though.

    edit:
    OK, I'm pretty sure that 8.9447 is the right answer (though I don't know how to work that out without a calculator).
    I'mpretty sure it's the cubing everything that seems to upset things, but I have no clue how else I could do this.
    huh...i figured you have to cube it if you wanna get rid of the radicals and all. :\
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  4. Feb 7th, 2004, 07:40 PM #4
    da_silvy
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    your expansion of [3 + (x - 9)^(1/3)]^3 is incorrect

    [3 + (x - 9)^(1/3)]^3 <> 27 + x - 9
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  6. Feb 7th, 2004, 07:50 PM #5
    dolor
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    Originally posted by da_silvy
    your expansion of [3 + (x - 9)^(1/3)]^3 is incorrect

    [3 + (x - 9)^(1/3)]^3 <> 27 + x - 9
    omg...i see it now. thank you so much.
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  7. Feb 7th, 2004, 07:59 PM #6
    dolor
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    Originally posted by Acidic
    well, my calculator seems to dissagree, I'm trying to work out why though.

    edit:
    OK, I'm pretty sure that 8.9447 is the right answer (though I don't know how to work that out without a calculator).
    I'mpretty sure it's the cubing everything that seems to upset things, but I have no clue how else I could do this.
    i reworked it w/the correct cubic expansion...and i got x=8.94427 so x^2 = 8.94427^2 ..whatever that is
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  8. Feb 7th, 2004, 08:12 PM #7
    da_silvy
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    i got x^2 = 80

    same as you
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  9. Feb 7th, 2004, 09:00 PM #8
    dolor
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    Originally posted by da_silvy
    i got x^2 = 80

    same as you

    good show
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  11. Feb 8th, 2004, 11:45 AM #9
    dolor
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    you guys know an easier way to do this? cuz the cubic expansion is ugly as you know what..
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  12. Feb 9th, 2004, 06:54 PM #10
    DiGiTaIErRoR
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    That made my computer think for a bit....

    x^2=80 though

    or x=8.944
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