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Math Help - Calc integral problem

  1. #1
    Junior Member
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    Calc integral problem

    hi everyone!

    i know how to integrate the following using substitution, but how does one do this using parts?

    the integral of ln(x)/x

    So this is what I have so far.

    u = ln(x), u' = (1/x), v' = (1/x), v = ln(x)

    Then, I got:

    ln squared x - the integral of (1/x)ln(x)

    I guess my problem starts with how to integrate the (1/x)ln(x)

    any help is appreciated!!!
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  2. #2
    is up to his old tricks again! Jhevon's Avatar
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    You were right so far, now all you have to do is transpose (solve) the equation for int[(lnx)/x]

    Notice that you had

    int[(lnx)/x]dx = (lnx)^2 - int[(lnx)/x]dx + C

    solving for int[(lnx)/x]dx, we obtain:

    2*int[(lnx)/x]dx = (lnx)^2 + C

    divide by 2 on both sides:

    int[(lnx)/x] dx = [(lnx)^2]/2 + C

    You should make a note of this technique, it comes in handy when you're integrating things that end up going in a loop and coming back to the problem you started in the first place. Functions like e^x * (any trig function) (for instance, the same thing happens if you try to do int(e^x*sin(x) dx) by parts) do something like this when you try to integrate them by parts, so all you do is just solve for the thing you want, don't keep integrating by parts over and over.
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