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Homework Help: Integration of Laplace transform

  1. Apr 10, 2013 #1
    1. The problem statement, all variables and given/known data

    Find the inverse transform of the function

    [tex] F(s) = log\frac{s-2}{s+2} [/tex]

    2. Relevant equations

    [tex] L(\frac{f(t)}{t}) = \int^{∞}_{s}F(x)dx [/tex]

    [tex] f(t) = tL^{-1}(\int^{∞}_{s}F(x)dx)[/tex]

    3. The attempt at a solution

    I missed the lecture on this and while I was able to figure out differentiation of transforms I've been unable to get this right. The textbook introduces the definition with the conditions necessary for the Laplace transform of f(t)/t, states the two formulas above, gives one example and then finishes the section.

    [tex] L^{-1}(log(\frac{s-2}{s+2})) [/tex]

    [tex] tL^{-1}(\int^{∞}_{s}log(\frac{x-2}{x+2}) dx) [/tex]

    The main problem I'm having here is with the integrand.

    [tex] log(x-2) - log(x+2) [/tex]

    I can easily integrate any of the two with integration by parts. Since both parts are similar, I'll just pick log(s-2).

    Letting u = log(x-2) and dv = 1

    [tex] [xlog(x-2)]^{∞}_{s} - \int^{∞}_{s}\frac{x}{x-2} [/tex]

    The amount of problems coming up by doing this is making me think I'm applying the Laplace transform wrong. If I go back now and look at the entire thing:

    [tex] tL^{-1}(\int^{∞}_{s}\frac{-x}{x-2} dx + [xlog(x-2)]^{∞}_{s} - \int^{∞}_{s}log(x+2) dx ) [/tex]

    The first term integrates into x + 2log(x-2). I have no idea how to apply the inverse Laplace tranform to a logarithm though and judging by the previous sections and problems, I'm not supposed to.

    Even if I figured out how to somehow apply the inverse Laplace transform to the first term, the second term diverges when evaluating the limits of integration.

    I figure I'm going at this completely wrong somewhere in the beginning, but where?
    Last edited: Apr 10, 2013
  2. jcsd
  3. Apr 11, 2013 #2


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    Gold Member

    The integral for the inverse transform of a logarithm is hard to do, but the integral for ##1/(s-a)## is much easier. Try to find a formula for the inverse transform of ##F'(s)## if ##F(s)## is the Laplace transform of ##f(t)##. You should be able to find the right formula by differentiating the usual integral expression for ##F(s)##.
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