Deriving some Laplace transforms

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Homework Help Overview

The discussion revolves around finding the Laplace transforms of specific functions, including \( f(t) = e^{at} \cos(bt) \) and \( f(t) = t^n \) where \( n \) is a positive integer. Participants are exploring the integration process involved in deriving these transforms.

Discussion Character

  • Exploratory, Mathematical reasoning, Problem interpretation

Approaches and Questions Raised

  • The original poster attempts integration by parts but expresses difficulty with the process, noting that it seems to go on indefinitely. They question how to effectively apply integration by parts to the Laplace transform of the first function.
  • Some participants suggest repeating the integration by parts and collecting terms to simplify the problem, referencing a similar integral as an example.
  • There is a question about which integral is of interest in the context of the original problem.

Discussion Status

Participants are actively engaging with the problem, offering guidance on the integration process and discussing the structure of the integrals involved. There is an exploration of different approaches to the integration by parts technique, but no consensus has been reached on a specific method or solution.

Contextual Notes

The original poster indicates that the integration by parts approach is challenging, suggesting that there may be constraints or assumptions affecting their understanding of the problem. The discussion also hints at the complexity of the integrals involved in the Laplace transform derivation.

Jamin2112
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Homework Statement



Find the Laplace transform of each of the following functions:

...

2. f(t)=eatcos(bt)

3. f(t)=tn, were n is a positive integer

Homework Equations



As you well know, taking the Laplace of f(t) means ∫f(t)e-stdt from 0 to ∞

The Attempt at a Solution



These problems are tripping me up, since integration by parts goes on forever.

∫eatcos(bt)e-stdt =
∫et(a-s)cos(bt)dt =

I suppose I should call et(a-s) "dv" (?)
----> v = t(a-s)/(a-s)
----> u = cos(bt)
----> du = -bsin(bt) dt

Still, I don't see where this gets me. Help, please!
 
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Do the integration by parts again, and then collect the integral you are interested in on one side.

This is like the following simpler integral:

[tex]\int e^x \sin x dx[/tex]
[tex]\int e^x \sin x dx = -e^x \cos x - \int -e^x \cos x dx[/tex] (one IBP)
[tex]\int e^x \sin x dx = -e^x \cos x + e^x \sin x - \int e^x \sin x dx[/tex] (second IBP)
[tex]2 \int e^x \sin x dx = e^x \sin x - e^x \cos x[/tex] (collect like terms)
etc...
 
hgfalling said:
Do the integration by parts again, and then collect the integral you are interested in on one side.

This is like the following simpler integral:

[tex]\int e^x \sin x dx[/tex]
[tex]\int e^x \sin x dx = -e^x \cos x - \int -e^x \cos x dx[/tex] (one IBP)
[tex]\int e^x \sin x dx = -e^x \cos x + e^x \sin x - \int e^x \sin x dx[/tex] (second IBP)
[tex]2 \int e^x \sin x dx = e^x \sin x - e^x \cos x[/tex] (collect like terms)
etc...

In the case of this particular problem, what integral am I interested in?
 
Well, you want

[tex]\int_{0}^{\infty} e^{t(a-s)} \cos (bt) dt[/tex]

which is related, but with, you know, constants and things.
 

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