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Math Help - Natural log, solve for x, undefined?

  1. #1
    Junior Member
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    Natural log, solve for x, undefined?

    If you solve this eq for x: \ln (5-x^2)=4
    you get:
    x=\sqrt{5-e^4}}

    but that's not defined, is it?

    Is it still a correct answer? Since obviuosly if you insert the above for x, you will get 4.
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  2. #2
    MHF Contributor Unknown008's Avatar
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    No, I don't think you did something wrong...

    5-x^2 = e^4 \approx 54.6

    It means that x^4 must be negative, which we all know can only be positive since the power is even. There are no real solutions.
    Last edited by Unknown008; November 14th 2010 at 09:45 AM. Reason: Typo. I put 6 instead of 5
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