Having difficulty finding the inverse laplace transform

  • #1
Having difficulty finding the inverse laplace transform!!

Hello everyone, I am really stuck on finding the inverse Laplace transform for this:

[tex]f(s)=\frac{5se^{-3s} - e^{-3s}}{s^{2}-4s+17}[/tex]

Heres my reasoning: I feel that I should rewrite the denominator in some kind of form such as (s-2)^2 + 13, and note the similarity with some of the problems ive been doing before, however its that 13 that is bothering me! Its not something you can take the squareroot of--- and also in addtion, I tried factoring out e^-3s on top and splitting this into two equations, but its this denominator that I absolutely despise.

Any help with finding the right method would be greatly appreciated thank you!!!
 
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Answers and Replies

  • #2
arildno
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I've forgotten just about every transform formula I knew (and I'm not in the mood rederiving them), but you may write [tex]13=(\sqrt{13})^{2}[/tex]

if that helps..
 
  • #4
arildno
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Well, what about rewriting:
[tex]e^{-3s}=e^{-6}e^{-3(s-2)}[/tex]

Then you would get one expression on the form:
[tex]k\frac{e^{-3w}}{w^{2}+a^{2}},k=-e^{-6},w=s-2,a=\sqrt{13}[/tex]

Is this a familiar transform in w?
 
  • #5
Tom Mattson
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The exponentials are due to shifts in the t-domain:

L{f(t-T)}=e-sTF(s).

Just find the inverse transform of the rational functions of s, and then let the t-domain functions be delayed by the appropriate amount, given by the coefficient in the exponents of the exp functions.
 
  • #6
interesting let me see if i can get anywhere with that...
 
  • #7
Uh oh, you see, I cannot split the denominator into two linear factors like the book seems to be doing with most of the problems---- and because I cant, i dont know how to proceed like the examples do... Im sorry if im a bit slow but we just started Laplace and this is a challenge problem id like to know to prepare.
 
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  • #8
Well I cannot solve it but thanks for your help anyway...
 
  • #9
Tom Mattson
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Theelectricchild said:
Uh oh, you see, I cannot split the denominator into two linear factors like the book seems to be doing with most of the problems---- and because I cant, i dont know how to proceed like the examples do... Im sorry if im a bit slow but we just started Laplace and this is a challenge problem id like to know to prepare.
You could factor the denominator into 2 linear factors, but the roots of that polynomial are complex. It would be better instead to complete the square in the denominator. The solution will be t-shifted, exponentially damped sines and cosines.
 

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