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Integration by parts (Laplace transform)

  1. Mar 31, 2006 #1
    First off, I hope these images show up - I don't have time to figure out this latex stuff atm, so it's easier just to throw the formulae together in openoffice.

    I'm working on the Laplace Transform for

    Which is obviously

    Now, in case somebody brings it up, I know that we can look up the answer in the table of basic Laplace transforms as

    But if I could get off that easy, I wouldn't be asking this question, would I? ;)

    My main problem is that I'm fairly rusty on integration, so I'm probably missing something small. I'd picked out
    and was attempting the tabular method, but I'm not getting anywhere. It's looking to me like no matter what you pick for u or dv you'll be doing integration by parts forever, since you're stuck with e^t and sin.
    Just need some hints to kick start the thought process.
  2. jcsd
  3. Mar 31, 2006 #2
    The usual trick when it comes to integrating a product of an exponential and a trig function by parts is that you will get the original integral again. So for example if we let the original integral be I then we will reach a step that looks like:
    I = (some stuff) + kI => I = (some stuff)/(1 - k)

    Maybe this could help you.
  4. Mar 31, 2006 #3
    I haven't actually written the working out but I would try...

    e^{ - st} t\sin \left( {at} \right) = t\left( {e^{ - st} \sin \left( {at} \right)} \right)

    The t is quite pesky so get rid of it by using parts, differentiate the t and integrate the exponential and sine product (*). You'll be left with some complicated product (which you don't need to deal with) plus or minus some integral involving products of exponentials and sines or cosines. That should be easy to integrate.

    (*) integrating the product of an exponential and sine (or cosine) should be standard procedure considering the level that you're at.
  5. Mar 31, 2006 #4
    Yep, that's the ticket. I figured it was right under my nose.
  6. Apr 1, 2006 #5


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    A shortcut that is also much easier is to use the complex exponential:
    [tex]\int_0^\infty te^{-(s-ia)t}dt[/tex]
    Your integral is just the imaginary part of this. As a side bonus you also get the Laplace transform of [itex]t\cos(at)[/itex] by taking the real part.
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