Fin inverse Laplace Transform of F(s)=e^(−s) arctan ((s+4)/((s+4)^2+4))

albamarronpe

albamarronpe

Answered question

2022-10-01

Inverse Laplace Transform of F ( s ) = e s arctan ( s + 4 ( s + 4 ) 2 + 4 )

Answer & Explanation

Emilia Boyle

Emilia Boyle

Beginner2022-10-02Added 10 answers

Once you've dealt with the time and frequency shifts, the problem reduces to finding the inverse transform of
G ( s ) = arctan ( s s 2 + 4 )
The arctan outer function might suggest you use the derivative property
L 1 { G ( s ) } = t g ( t )
By the chain rule, we have
G ( s ) = 1 1 + s 2 ( s 2 + 4 ) 2 4 s 2 ( s 2 + 4 ) 2 = 4 s 2 ( s 2 + 4 ) 2 + s 2 = 4 s 2 s 4 + 9 s 2 + 16
The denominator here is a quadratic in s 2 with two roots: 9 ± 17 2 . Since they are both negative we can write
s 4 + 9 s 2 + 16 = ( s 2 + 9 17 2 ) ( s 2 + 9 + 17 2 ) = ( s 2 + a 1 2 ) ( s 2 + a 2 2 )
Applying partial fraction decomposition
4 s 2 ( s 2 + a 1 2 ) ( s 2 + a 2 2 ) = A s + B s 2 + a 1 2 + C s + D s 2 + a 2 2
You will quickly find A=C=0 since the LHS is an even function. The remaining two constants are up to you to solve.
Taking the inverse transform of G′(s) gives
g ( t ) = B sin ( a 1 t ) a 1 t D sin ( a 2 t ) a 2 t

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