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Math Help - logarithmic integration help please

  1. #1
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    logarithmic integration help please

    i'm scratching my head with this one:

    evaluate the indefinite integral:

    the integral of 2dx/(xln(5x))

    so what i do is pull the 2 in front of the integral

    use u-substitution

    u=ln5x

    du = 1/5x *5 dx = 1/x dx

    so i get

    2* integral of 1/u du

    which gives me

    2*lnu + C

    and substituting back in gives me

    2*ln(ln(5x))+C

    but apparently thats not right . can anyone tell me where i'm going wrong?
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  2. #2
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    Quote Originally Posted by mthomas1 View Post
    i'm scratching my head with this one:

    evaluate the indefinite integral:

    the integral of 2dx/(xln(5x))

    so what i do is pull the 2 in front of the integral

    use u-substitution

    u=ln5x

    du = 1/5x *5 dx = 1/x dx

    so i get

    2* integral of 1/u du

    which gives me

    2*lnu + C

    and substituting back in gives me

    2*ln(ln(5x))+C

    but apparently thats not right . can anyone tell me where i'm going wrong?
    I get the same answer as you and so does wolframalpha: integral of 2dx/(xln(5x)) - Wolfram|Alpha

    Is the correct answer given?
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  3. #3
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    Feb 2010
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    no, which is what is so frustrating. its an online assignment in which you must put in the correct answer, and when i enter that answer it says it is wrong. (btw i don't add the C, so the C is already given. I just put 2*ln[ln(5x)]
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