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Math Help - Calculus homework problem

  1. #1
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    Calculus homework problem

    I'm having a little trouble with this calculus problem I got for homework. I've been working on it for over an hour and haven't gotten anywhere with it.

    Solve for x: ln(x) + ln(x-4) = 5

    The best I could come up with was raising both sides to e to get rid of the natural logs, but then ended up with x^2 - 4x = e^5. Any advice/answers?
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  2. #2
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    x^2 - 4x - e^5 = 0

    Does the quadratic formula ring any bells ?
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  3. #3
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    Yeah, I got to that, but then how do you solve for x? The QF only works for ax^2 + bx + c, doesn't it? That would make it a bit harder since the e is raised to the 5th. I could do it if it was raised to the 4th, but since it's to the 5th I've got nothing.

    Or did you mean use the e^5 as the c in the QF?
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  4. #4
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    e^5 is just the constant term, so yes you can use the quadratic formula and you can use it as a "c".
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  5. #5
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    Quote Originally Posted by Chop Suey View Post
    e^5 is just the constant term, so yes you can use the quadratic formula and you can use it as a "c".
    Ok, thanks. That makes more sense now.
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  6. #6
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    e^a where a is any real number is a constant, for future reference
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