Find inverse laplace transform of f(s)=(1)/((s−2)^2+9)

Camila Brandt

Camila Brandt

Answered question

2022-09-20

Find inverse laplace transform of f ( s ) = 1 ( s 2 ) 2 + 9
Here is what I have gotten from partial fractions. I observed that s 2 4 s + 13 is irreducible (doesn't have real roots).
1 ( s 2 ) 2 + 9 = 1 s 2 4 s + 13 = A s + B s 2 4 s + 13
A=0, B=1
The corresponding value in my table is e a t sin ( b t ) for the corresponding laplace transform b ( s a ) 2 + b 2
so plugging in A and B, I get e 0 t sin ( 1 t ) = sin ( t ). However my textbook answer is 1 3 e 2 t sin 3 t

Answer & Explanation

Mackenzie Lutz

Mackenzie Lutz

Beginner2022-09-21Added 13 answers

Remember that
L 1 { F ( s a ) } = e a t L 1 { F ( s ) } and
L 1 { b s 2 + b 2 } = sin ( b t )
So,
L 1 { 1 ( s 2 ) 2 + 9 } = 1 3 L 1 { 3 ( s 2 ) 2 + 3 2 } = 1 3 e 2 t sin ( 3 t )
shaunistayb1

shaunistayb1

Beginner2022-09-22Added 4 answers

F ( s ) = 1 ( s 2 ) 2 + 3 2 3 F ( s ) = 3 ( s 2 ) 2 + 3 2
Substitute s 2 = s :
3 F ( s + 2 ) = 3 ( s ) 2 + 3 2
3 L 1 F ( s + 2 ) = L 1 ( 3 ( s ) 2 + 3 2 )
3 L 1 F ( s + 2 ) = sin ( 3 t )
Finally L 1 ( F ( s + a ) ) = e a t f ( t ) :
3 e 2 t f ( t ) = sin ( 3 t ) f ( t ) = e 2 t 3 sin ( 3 t )

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