F(S)=(−S+11)/(S^2−2S−3) How do I find f(t)? What is a good strategy for attacking these types of problems?

klesstilne1

klesstilne1

Answered question

2022-11-10

F ( S ) = S + 11 S 2 2 S 3
How do I find f(t)? What is a good strategy for attacking these types of problems?

Answer & Explanation

Lillianna Salazar

Lillianna Salazar

Beginner2022-11-11Added 22 answers

Factor the denominator:
s 2 2 s 3 = ( s 3 ) ( s + 1 )
Then note that
s + 11 ( s 3 ) ( s + 1 ) = A s 3 + B s + 1
where
A + B = 1 and A 3 B = 11
so that A=2 and B=−3. Thus
F ( s ) = 2 s 3 3 s + 1
Now know that the inverse Laplace transform of 1 / ( s + a ) is e a t when s>−a.
You should be able to see this from
0 d t e a t e s t = 1 s + a
You should be able to take it from here.
Alternatively, you can use the residue theorem if you know some complex analysis. Here, you see that the poles of F are at s=−1 and s=3. The ILT is the sum of the residues of F ( s ) e s t at the poles:
Res s = 1 s + 11 ( s 3 ) ( s + 1 ) e s t = 12 4 e t
Res s = 3 s + 11 ( s 3 ) ( s + 1 ) = 8 4 e 3 t

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