Finding the laplace transform of a squared function f(t)=t^2 cos^2(t)

Alice Chen

Alice Chen

Answered question

2022-11-04

Find the laplace transform of
f ( t ) = t 2 c o s 2 ( t )

Answer & Explanation

hitturn35

hitturn35

Beginner2022-11-05Added 20 answers

Hint:
L ( t 2 f ( t ) ) = + d 2 F ( s ) d s 2
And you can rewrite the equation to the following:
c o s 2 t = 1 2 ( c o s ( 2 t ) + 1 )
Applying this gives:
L ( t 2 c o s 2 t ) = L ( t 2 1 2 ( c o s 2 t + 1 ) )
= 1 2 L ( t 2 c o s 2 t ) + 1 2 L ( t 2 )
This gives:
1 2 d 2 ( s s 2 + 4 ) d s 2 + 1 s 3 = s ( s 2 12 ) ( s 2 + 4 ) 3 + 1 s 3
ajakanvao

ajakanvao

Beginner2022-11-06Added 4 answers

Notice, by the definition of the Laplace transform:
L t [ t 2 cos 2 ( t ) ] ( s ) = 0 t 2 cos 2 ( t ) e s t   d t
Now, for the integral:
0 t 2 cos 2 ( t ) e s t   d t = 1 2 [ 0 t 2 cos ( 2 t ) e s t   d t + 0 t 2 e s t   d t ]
Use integration by parts:
0 t 2 cos ( 2 t ) e s t   d t =
[ t 2 e s t ( 2 sin ( 2 t ) s cos ( 2 t ) ) 4 + s 2 ] 0 2 s 2 + 4 0 t e s t ( 2 sin ( t ) s cos ( 2 t ) )   d t
0 t 2 e s t   d t = [ t 2 e s t s ] 0 + 2 s 0 t e s t   d t

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