Laplace Transform solution verification: ddot(y)+2y=2e^t => 1/3 cos(sqrt(2)t)−2/(3sqrt2) sin(sqrt(2)t)+2/3 e^t?

fabler107

fabler107

Answered question

2022-11-02

Laplace Transform solution verification: y ¨ + 2 y = 2 e t 1 3 cos ( 2 t ) 2 3 2 sin ( 2 t ) + 2 3 e t ?

Answer & Explanation

luthersavage6lm

luthersavage6lm

Beginner2022-11-03Added 22 answers

I am not sure if you used partial fractions and table to solve the problem so I will post a solution using the inverse Mellin transform. The Laplace transform of the ODE is
s 2 Y ( s ) s y ( 0 ) y ˙ ( 0 ) + 2 Y ( s ) = 2 0 e t e s t d t Y ( s ) ( s 2 + 2 ) s = 2 s 1 Y ( s ) = 2 ( s 1 ) ( s 2 + 2 ) + s s 2 + 2
Then by the inverse Mellin transform, we have that the poles are at s = ± 2 i , 1
y ( t ) = 1 2 π i γ i γ + i [ 2 e s t ( s 1 ) ( s 2 + 2 ) + s e s t s 2 + 2 ] d s = Res = lim s 1 ( s 1 ) 2 e s t ( s 1 ) ( s 2 + 2 ) + lim s 2 i ( s 2 i ) 2 e s t ( s 1 ) ( s 2 + 2 ) + lim s 2 i ( s + 2 i ) 2 e s t ( s 1 ) ( s 2 + 2 ) + lim s 2 i ( s 2 i ) s e s t s 2 + 2 + lim s 2 i ( s + 2 i ) s e s t s 2 + 2 = 2 e t 3 2 3 cos ( t 2 ) 2 3 sin ( t 2 ) + cos ( t 2 ) = 2 e t 3 + 1 3 cos ( t 2 ) 2 3 sin ( t 2 )
which is of no surprise the solution you obtained.

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