Without using the shift formula prove that Laplace of f(t)=e^{-at}\cos wt is \frac{w^2}{(s+\alpha)^2+w^2}

CMIIh

CMIIh

Answered question

2021-09-05

Without using the shift formula prove that Laplace of
f(t)=eatcoswt is w2(s+α)2+w2

Answer & Explanation

SchepperJ

SchepperJ

Skilled2021-09-06Added 96 answers

Step 1
Given,
f(t)=eatcoswt
To prove that, L(f(t))=s+α(s+α)2+w2
By definition,
L(f(t))=0esteatcoswtdt
=0e(α+s)tcoswtdt
Step 2
Let
I=0e(α+s)tcoswtdt
=[coswt×e(α+s)t(α+s)]00wsinwt×e(α+s)t(α+s)dt
=1α+sw(α+s){[sinwt×e(α+s)t(α+s)]00wcoswt×e(α+s)t(α+s)dt}
=1α+sw(α+s)[w(α+s)0e(α+s)tcoswtdt]
=1α+sw2(α+s)2I
Therefore, the Laplace transform is,
L(f(t))=1s+α[(s+α)2(s+α)2+w2]
=s+α

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