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Math Help - Solve for X

  1. #1
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    Exclamation Solve for X

    Solve for x: log(2x+20)=-2
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  2. #2
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    Quote Originally Posted by VNVeteran View Post
    Solve for x: log(2x+20)=-2
    hope this is right.

    log_{10} (2x + 20) = -2

    2x + 20 = \frac{1}{10^2}

    2x + 20 = \frac{1}{100}

    200x + 2000 = 1

    x = -\frac{1999}{200}
    Last edited by mr fantastic; November 20th 2009 at 08:04 PM. Reason: Fixed a latex subscript and added some latex tags.
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  3. #3
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    Hello
    \log(2x+20)=-2\Rightarrow 2x+20=\exp(-2)\Leftrightarrow x =\frac{\exp(-2)-20}{2}=\frac{1-20\exp(2)}{2\exp(2)}\approx -9,93233.
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  4. #4
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    Quote Originally Posted by ukorov View Post
    hope this is right.

    log_{10} (2x + 20) = -2

    2x + 20 = \frac{1}{10^2}

    2x + 20 = \frac{1}{100}

    200x + 2000 = 1

    x = -\frac{1999}{200}
    Same way I'd have done it.
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  5. #5
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    Wrong answer Raoh , because log is a base 10 logarithm not base e , base e log is the " ln " so ukorov answer is the right answer
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  6. #6
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    Quote Originally Posted by mosta86 View Post
    Wrong answer Raoh , because log is a base 10 logarithm not base e , base e log is the " ln " so ukorov answer is the right answer
    well..he should have written \log_{10}(2x+20)=-2,i assumed \log is the natural logarithm.
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  7. #7
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    yes . but as it is by convention log is log base 10
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  8. #8
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    Quote Originally Posted by mosta86 View Post
    yes . but as it is by convention log is log base 10
    well..in where i live u should precise.
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  9. #9
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    lol .. ok but check you calculator or any calculator , the log on it is base 10 , check all the references and books , log is base 10 ,, any other base you should specify . > Mostafa Ghalayini
    B.E Computer and Communication engineering
    M.S.E Computer and Communication engineering
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  10. #10
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    didn't i say "I ASSUMED" ?
    .END OF MY ASSUMPTION.
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  11. #11
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    Actually, it is common practice in advanced math books (Calculus or higher) to use just "log" to mean natural logarithm since common logarithms are seldom if ever used. The practice in more basic books is use "log" to mean common logarithms and "ln" to mean natural logarithms. Here, the question was posted in "Pre-Calculus" so I would have assumed that common logarithms were meant, but it is certainly not true that "log" always means common logarithm.

    In any case, it would have been better if the original post had specified which logarithm was intended and if the responders had said which convention they were using!
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