Is My Laplace Transform Solution Correct?

  • Context: Undergrad 
  • Thread starter Thread starter CentreShifter
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Discussion Overview

The discussion revolves around the correctness of a solution to an initial value problem (IVP) using the Laplace transform. Participants are examining the formulation of the differential equation and the application of the Laplace transform method.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant presents a solution to the IVP but questions whether there is a typo in the book regarding the differential equation.
  • Another participant asks for clarification on the exact form of the differential equation, suggesting that it may be missing from the original post.
  • A third participant proposes a trial solution using the method of undetermined coefficients, indicating that the left-hand side simplifies to zero, which raises questions about the initial conditions and the formulation of the problem.
  • A later reply acknowledges the oversight regarding the missing equation and emphasizes that the problem should be solved using Laplace transforms rather than undetermined coefficients.
  • Participants discuss the implications of the missing equation on the solution process and the need for clarity in the problem statement.

Areas of Agreement / Disagreement

Participants generally agree that the differential equation was not clearly stated initially, leading to confusion. There is no consensus on the correctness of the original solution due to the ambiguity in the problem formulation.

Contextual Notes

The discussion highlights the importance of precise problem statements in mathematical contexts, particularly when applying specific solution methods like the Laplace transform. The lack of clarity in the differential equation affects the evaluation of the proposed solution.

CentreShifter
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I think there may be a typo in the book, I'm pretty sure I'm doing this correctly.

Use the Laplace transform to solve the IVP: y"-6y'+9y=t; y(0)=0, y'(0)=0

My solution is e[tex]^{3t}[/tex](1/9*t - 2/27) + 1/9*t + 2/27.

Can someone quickly solve it again for me?
 
Last edited:
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CentreShifter said:
I think there may be a typo in the book, I'm pretty sure I'm doing this correctly.

Use the Laplace transform to solve the IVP: y"-6y'+9y; y(0)=0, y'(0)=0

My solution is e[tex]^{3t}[/tex](1/9*t - 2/27) + 1/9*t + 2/27.

Can someone quickly solve it again for me?

Hi CentreShifter! :smile:

y"-6y'+9y = what? :confused:
 
CentreShifter said:
I think there may be a typo in the book, I'm pretty sure I'm doing this correctly.

Use the Laplace transform to solve the IVP: y"-6y'+9y; y(0)=0, y'(0)=0

My solution is e[tex]^{3t}[/tex](1/9*t - 2/27) + 1/9*t + 2/27.

Can someone quickly solve it again for me?
Well, what is your differential EQUATION??

Is it: y"-6y'+9y=0 ?

First, we identify that [itex]Ae^{3t}[/tex] is, indeed, a double root-solution of the IVP<br /> <br /> In order to find a second solution, we try with:<br /> [tex]Bte^{3t}[/tex]<br /> Inserting this trial solution into our equation yields:<br /> [tex](6Be^{3t}+9Bte^{3t})-6(Be^{3t}+3Bte^{3t})+9Bte^{3t}=0[/tex]<br /> Note that simplification of the left-hand side yields:<br /> [tex]0=0[/tex]<br /> <br /> This is precisely what you should have, since you now have two arbitrary parameters, A og B, by which you may adjust your general solution, [tex]y=Ae^{3t}+Bte^{3t}[/tex], to the initial conditions.<br /> <br /> (Note that this will yield you y=0 as your solution, do you now realize WHY you must state precisely what your diff. eq. actually was?[/itex]
 
You are both absolutely correct. I was the end of my study session, there should definitely be an equation there. I'll be posting it as soon as I can get to the book.

@arildno - I know this doesn't help right now, but the problem is to be solved using Laplace transforms, not undetermined coefficients (although I suppose t doesn't really matter as long as the solution is correct).

Edit: I have fixed the equation in the first post. It's now correct.
 
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