Find Laplace Transform of the following: sqrt(1+sin(4t))

Hagman7v

Hagman7v

Answered question

2022-09-23

Find Laplace Transform of the following: 1 + sin ( 4 t )

Answer & Explanation

Claire Larson

Claire Larson

Beginner2022-09-24Added 10 answers

The function 1 + sin a t is periodic with period T = 2 π / a and the first zero at 3 π / 2 a. Following your steps, we can write this as
1 + sin a t = cos 2 a t 2 + sin 2 a t 2 + 2 sin a t 2 cos a t 2 = | cos a t 2 + sin a t 2 | .
Using the Laplace transform of a periodic function, we split the integral into two pieces. This is necessary because of the absolute value sign. To get rid of it in the region [ 3 π / 2 a , 2 π / a ], we have to take the negative of the function.
I = 0 1 + sin a t e s t d t = 1 1 e 2 π s / a 0 2 π / a | cos a t 2 + sin a t 2 | e s t d t = 1 1 e 2 π s / a [ 0 3 π / 2 a ( cos a t 2 + sin a t 2 ) e s t d t 3 π / 2 a 2 π / a ( cos a t 2 + sin a t 2 ) e s t d t ]
These integrals can be done by considering the integral
0 3 π / 2 a e i a t / 2 e s t d t
and summing the real and imaginary contributions. One should then obtain
L [ 1 + sin a t ] = 1 s 2 + a 2 / 4 ( s + a 2 + 2 a e 3 π s / 2 a 1 e 2 π s / a ) .
For a=4, we have
L [ 1 + sin 4 t ] = 1 s 2 + 4 ( s + 2 + 4 2 e 3 π s / 8 1 e π s / 2 ) .

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