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Laplace Transform

  1. Jun 10, 2012 #1
    1. The problem statement, all variables and given/known data
    Is there an easier way of solving this rather than doing the integral?
    Find the laplace transform of:

    [tex]t{e^{ - t}}u(t - 1)[/tex]



    2. Relevant equations



    3. The attempt at a solution
    [tex]\int_1^\infty {t{e^{ - t}}} {e^{ - st}}dt = \int_1^\infty {t{e^{ - t(1 + s)}}} dt = \frac{{{e^{ - (s + 1)}}}}{{{{(1 + s)}^2}}} + \frac{{{e^{ - (s + 1)}}}}{{1 + s}}[/tex]
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Jun 10, 2012 #2

    LCKurtz

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    Gold Member

    That integral isn't that tough, but if you prefer, you can use some of the theorems. For example, you have, if ##\mathcal L(f(t)) = F(s)## then ##\mathcal L(tf(t))=-F'(s)##. Also, ##\mathcal L(e^{at}f(t) = F(s-a)## and ##\mathcal L(u(t-1))=\frac{e^{-s}}{s}##.

    So starting with ##\mathcal L(u(t-1))=\frac{e^{-s}}{s}= F(s)##, then ##\mathcal Le^{-t}u(t-1)=F(s+1) =\frac{e^{-(s+1)}}{s+1}## and $$L(te^{-t}u(t-1)) = -\frac d {ds}\left(\frac{e^{-(s+1)}}{s+1}\right)=\frac{e^{-s+1}(s+1)+e^{-s+1}}{(s+1)^2}$$which is the same answer.
     
  4. Jun 10, 2012 #3
    True - it isn't that tough. But I am not so keen on integration by parts if I can avoid it.
    Thanks for the reply.
     
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