Method of Variation of Parameters

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Discussion Overview

The discussion revolves around the method of variation of parameters in solving second-order linear differential equations, particularly focusing on the need for two solutions of the homogeneous equation to apply the method effectively. Participants explore how to find the second solution when only one is known, and they discuss specific techniques such as the Reduction of Order method.

Discussion Character

  • Technical explanation
  • Homework-related
  • Debate/contested

Main Points Raised

  • One participant notes the requirement of two solutions, y1 and y2, to apply the method of variation of parameters and describes the equations involved.
  • Another participant suggests that the textbook may provide a method to find the second solution of the homogeneous differential equation, emphasizing the necessity of both solutions for applying the variation of parameters.
  • A participant questions the approach to writing the characteristic equation due to the variable coefficients in the differential equation, expressing confusion about how to proceed with only one equation for two unknowns.
  • One participant proposes that the Reduction of Order method could be a viable approach to find the second solution, seeking confirmation from others.
  • A later reply confirms the suggestion regarding the Reduction of Order method as a correct approach.

Areas of Agreement / Disagreement

Participants generally agree on the necessity of finding a second solution to apply the method of variation of parameters, but there is some uncertainty regarding the specific methods to achieve this, particularly in relation to the characteristic equation and the use of the Reduction of Order method.

Contextual Notes

Participants express limitations in their understanding of how to derive the second solution given the variable coefficients in the differential equation, as well as the dependence on the known solution.

Augmented
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Allright, I understand that we need two solutions to be able to apply the method like [tex]y_{1}[/tex] and [tex]y_{2}[/tex]

Problem gives 1 of them or let's you find only that 1 solution. But I can't apply the method since I don't have the other solution. The method I know is:

[tex]u_{1}'(x)y_{1}(x)+u_{2}'(x)y_{2}=0[/tex]
[tex]u_{1}'(x)y_{1}'(x)+u_{2}'(x)y_{2}'=g(x)[/tex]

solve for the [tex]u_{1}'(x)[/tex] and [tex]u_{2}'(x)[/tex] and do the integrals, solve the problem.

This is the problem I'm tackling with:

Find a value of [tex]p[/tex] such that [tex]e^{px}[/tex] is a solution of
[tex]xy''+(x-1)y'-y=2x^{2}e^{-x}[/tex]

well i can find out that p=-1 and its correct I'm pretty sure. How can I handle the rest of it as i mentioned above?
Thanks.
 
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So first you need to do something else. Perhaps your textbook has this also: in a second-order homogeneous linear DE, given one solution to find the other one. As you note, you need to have both solutions of the homogeneous DE before you can apply the method of variation of parameters to get a solution of the inhomogeneous DE.
 
g_edgar said:
So first you need to do something else. Perhaps your textbook has this also: in a second-order homogeneous linear DE, given one solution to find the other one. As you note, you need to have both solutions of the homogeneous DE before you can apply the method of variation of parameters to get a solution of the inhomogeneous DE.

Should it be writing down characteristic equation ?? I see no way out with the [tex]x[/tex] and [tex]x-1[/tex] that are coefficients to [tex]y''[/tex] and [tex]y'[/tex]. As I write it like this:

[tex]xr^{2}+(x-1)r-1=0[/tex]
umm, 2 unknowns, 1 equation ? Maybe with the known solution helps here? How?
 
Allright, I think i found a clue: Reduction of Order method will let us find the other solution ? Please someone confirm it whether wrong or right. Thanks
 
Augmented said:
Reduction of Order

right
 

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