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Homework Help: Iterated Integration with a fraction

  1. Nov 15, 2011 #1
    I need help with Iterated Integration. I'm trying to integrate the following equation:

    ∫ylnx/x from e^y to y for dx, y>0

    (sorry about the messy equation; first time user)

    I know how to integrate ln, but the fraction has me thrown for a loop. I tried U-substitution, but the equation kept getting messier. To be clear, I'm out of practice with some of the alternate methods for integration, so I may not be considering something (I planned on practicing them over the summer, but I got a job and my time got eaten away).

    Please help.
     
  2. jcsd
  3. Nov 15, 2011 #2

    HallsofIvy

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    Did you try the rather obvious substitution: u= ln(x)?
     
  4. Nov 15, 2011 #3
    Yes; it's the fraction I'm having a problem with, though. I am not sure why, though. I've done them before. This semester though I haven't run into them often, and the ones I've seen were done in class as example problems (the teacher picks from his homework assignments).

    I do have integration tables in the back of the book. I'll try those and see if one of them matches.
     
  5. Nov 15, 2011 #4

    SammyS

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    Hello Millenniumf,

    Welcome to PF.

    The substitution suggested by HallsofIvy takes care of the "fraction" very nicely.

    [itex]\displaystyle \int\frac{\ln(x)}{x}dx[/itex]

    Let u=ln(x), then du = ?
     
    Last edited: Nov 15, 2011
  6. Nov 15, 2011 #5
    Huh, it's obvious now! u=ln(x), du=(1/x), then I just input e^y and y and complete the problem.

    I think my brain must have been fried from pulling an all-nighter getting my assignment completed; I've done more complex u-sub problems before with no complications.

    I think I better start earlier in the day with future assignments.

    Thank you for the help!
     
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