Second-order homogeneous linear differential equation

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Discussion Overview

The discussion revolves around solving the second-order homogeneous linear differential equation $y'' + 4y' + Ky = 0$ with a specific case where $K = 4$. Participants explore the implications of having a repeated root in the characteristic equation and the form of the general solution.

Discussion Character

  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • One participant presents the characteristic equation derived from the differential equation, noting that it simplifies to $(r + 2)(r + 2) = 0$, leading to a repeated root $r = -2$.
  • Another participant states that for a repeated root $r$, the general solution takes the form $y(x) = c_1 e^{rx} + c_2 x e^{rx}$.
  • A subsequent reply confirms the general solution as $y = C_1 e^{-2t} + C_2 te^{-2t}$, applying the previously mentioned formula for repeated roots.
  • One participant suggests using the reduction of order method as an exercise to verify the general solution form for repeated roots.

Areas of Agreement / Disagreement

Participants generally agree on the form of the general solution for the case of a repeated root, but there is uncertainty expressed by the first participant regarding the implications of having two identical solutions.

Contextual Notes

The discussion does not resolve the participant's concern about the learning implications of repeated roots, nor does it clarify the necessity of the reduction of order method in this context.

shamieh
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Consider the second-order homogeneous linear differential equation $y'' + 4y' + Ky = 0$

Find the general solution if $K = 4$

So here is what I have:

$r^2 + 4r + 4 = 0 $
=$(r + 2)(r+2)$
$r=-2$ ?

But I thought that you can't do this because you won't be learning anything new if you have two of the same solutions. I'm not sure what to do
 
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If you have a repeated root $r$, then the general solution is:

$$y(x)=c_1e^{rx}+c_2xe^{rx}$$
 
so $y = C_1e^{-2t} + C_2 te^{-2t}$
 
Yes...as an exercise you may wish to use the reduction of order method to prove the form of the general solution is as I gave above in the case of a repeated root for the characteristic equation. :D
 

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