How to work out the Laplace transform with respect to t of: sin(t) (d^2y)/(dt^2)

Howard Nelson

Howard Nelson

Answered question

2022-11-05

How to work out the Laplace transform with respect to t of:
sin ( t ) d 2 y d t 2
I know that the transform of sin ( t ) is a s 2 + a 2 , and transform of d 2 y d t 2 is s 2 F ( s ) s f ( 0 ) s f ( 0 )

Answer & Explanation

Taniyah Lin

Taniyah Lin

Beginner2022-11-06Added 14 answers

I assume y and its derivatives are exponentially bounded. Then using integration by parts twice, I get (for Re ( s ) sufficiently large)
y ( 0 ) + ( s 2 1 ) F ( s ) 2 s G ( s )
where F(s) and G(s) are the Laplace transforms of y ( t ) sin ( t ) and y ( t ) cos ( t ) respectively.
Nicholas Hunter

Nicholas Hunter

Beginner2022-11-07Added 3 answers

Explicit computation yiels
0 y ( t ) sin t e s t d t
= sin t e s t y ( t ) | 0 + 0 y ( t ) ( s e s t sin t e s t cos t ) d t
We see the first term is 0 if y is of exponential order. Assuming this we continue
= s e s t sin t y | 0 0 y ( s e s t cos t + sin t ( s 2 e s t ) ) d t y e s t cos t | 0 + 0 y ( e s t sin t + cos t ( s e s t ) ) d t
We have
L { y ( t ) sin t } = y ( 0 ) + ( s 2 1 ) 0 y e s t sin t d t 2 s 0 y cos t e s t d t

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