f(s)=(2s^2)/((s+3)(s^2+4)). How can I calculate the inverse Laplace transform of the following function to get the solution?

Krish Logan

Krish Logan

Answered question

2022-10-28

I want to solve this integral equation using Laplace:
Y ( t ) + 3 0 t Y ( t ) d t = 2 c o s ( 2 t )
if
L { Y ( t ) } = f ( s )
then,
f ( s ) + 3 f ( s ) s = 2 s s 2 + 4
doing some operations I obtain
f ( s ) = 2 s 2 s 3 + 3 s 2 + 4 s + 12
and using Ruffini in the denominator
f ( s ) = 2 s 2 ( s + 3 ) ( s 2 + 4 )
How can I calculate the inverse Laplace transform of the following function to get the solution?

Answer & Explanation

Claire Love

Claire Love

Beginner2022-10-29Added 14 answers

Using partial fractions, we can obtain
2 s 2 ( s + 3 ) ( s 2 + 4 ) = 8 ( s 3 ) 13 ( s 2 + 4 ) + 18 13 ( s + 3 ) = 1 13 ( 8 s s 2 + 2 2 12 2 s 2 + 2 2 + 18 s ( 3 ) ) .
Take the inverse we will obtain
1 13 ( 8 cos 2 t 12 sin 2 t + 18   e 3 t ) .
robbbiehu

robbbiehu

Beginner2022-10-30Added 5 answers

try the following:
f ( s ) = 2 s 2 ( s + 3 ) ( s 2 + 4 ) = A s + B s 2 + 4 + C s + 3
So A s ( s + 3 ) + B ( s + 3 ) + C ( s 2 + 4 ) = 2 s 2 , whence\
A + C = 2 3 A + B = 0 3 B + 4 C = 0
We can shortcut the solution a bit, as follows:
Plug in s=−3, we get: 13 C = 18, whence C = 18 13
Plug in s=0, we get 3 B + 4 C = 0, so B = 24 13
Then using the first equation, we get A = 8 13
So we have
f ( s ) = 8 13 s s 2 + 4 24 13 s 2 + 4 + 18 13 s + 3 = 1 13 ( 8 s s 2 + 4 12 2 s 2 + 4 + 18 s + 3 )
And at this point I can refer you to a table of Laplace transforms (which implicitly gives the termwise inverse Laplace transform).
f ( t ) = 1 13 ( 8 cos ( 2 t ) 12 sin ( 2 t ) + 18 e 3 t )

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