Find the inverse Laplace Transform of: ccL^(−1){(e^(−2s))/(s^3)}

vidamuhae

vidamuhae

Answered question

2022-11-16

Find the inverse Laplace Transform of: L 1 { e 2 s s 3 }

Answer & Explanation

Phiplyrhypelw0

Phiplyrhypelw0

Beginner2022-11-17Added 24 answers

Here's one way to approach the problem:
L 1 { e 2 s 1 s 3 } = L 1 { e 2 s } L 1 { 1 / s 3 } = δ ( t 2 ) 1 2 t 2 u ( t ) = 1 2 ( t 2 ) 2 u ( t 2 )
Where ∗ represents convolution.
Sophie Marks

Sophie Marks

Beginner2022-11-18Added 2 answers

Another approach is to use the inverse Laplace transform.
L 1 { F ( s ) } ( t ) = γ i γ + i F ( s ) e s t d s
For your problem, we have
L 1 { e 2 s / s 3 } ( t ) = γ i γ + i e s ( t 2 ) s 3 d s = Res { f ( s ) ; s j }
In order for convergence, the exponential terms needs to converge.
That is, s ( t 2 ) < 0 or t < 2. We can capture this with the unit step
U ( t 2 ) = { 0 , t < 2 1 , t > 2
We can use the series expansion to find the residue since the residue is the coefficient of the s 1 term.
e s ( t 2 ) s 3 = n = 0 [ s ( t 2 ) ] n n ! s 3 = 1 s 3 + t 2 s 2 + ( t 2 ) 2 2 s + O ( s n )
Therefore, the residue is ( t 2 ) 2 2 . Putting this all together, we have
L 1 { e 2 s / s 3 } ( t ) = ( t 2 ) 2 2 U ( t 2 )

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