How the partial fraction decomposition works for finding this Inverse Laplace Transform? (A)/((s−a)(s−r_1)(s−r_2))

Kailey Vargas

Kailey Vargas

Answered question

2022-09-10

I've been working to find inverse Laplace transform for the following :
A ( s a ) ( s r 1 ) ( s r 2 )
However, I'm getting stuck on the partial fraction decomposition. When I run the decomposition in Wolfram Alpha, it comes back as
A ( s r 1 ) ( a r 1 ) ( r 1 r 2 ) A ( s r 2 ) ( a r 2 ) ( r 2 r 1 ) + A ( a r 1 ) ( a r 2 ) ( s a )
Any thoughts on how this decomposition works?

Answer & Explanation

Marie Horn

Marie Horn

Beginner2022-09-11Added 12 answers

For a , r 1 , r 2 not equals
A ( s a ) ( s r 1 ) ( s r 2 ) = A ( 1 ( a r 1 ) ( a r 2 ) ( s a ) + 1 ( r 1 a ) ( r 1 r 2 ) ( s r 1 ) + 1 ( r 2 a ) ( r 2 r 1 ) ( s r 2 ) )
In case of equality ( for example a=r1 or r1=r2) the decomposition is different.
Take a simple example:
f ( s ) = 1 ( s 2 ) ( s 3 )
It is decomposed as
f ( s ) = A ( s 3 ) + B ( s 2 )
For A you calculate h ( s ) = ( s 3 ) f ( s ) A = h ( 3 ) = 1 . For B you calculate h ( s ) = ( s 2 ) f ( s ) B = h ( 2 ) = 1 Therefore we have:
f ( s ) = 1 ( s 3 ) 1 ( s 2 )
Nyasia Flowers

Nyasia Flowers

Beginner2022-09-12Added 1 answers

Calling
f ( s ) = A ( s a ) ( s r 1 ) ( s r 2 )
supposing that a r 1 r 2 we have by residues
f ( s ) = ( lim s a ( s a ) f ( s ) ) 1 s a + ( lim s r 1 ( s r 1 ) f ( s ) ) 1 s r 1 + ( lim s r 2 ( s r 2 ) f ( s ) ) 1 s r 2

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