Initial value problem Euler equation

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Homework Help Overview

The discussion revolves around solving an initial value problem involving an Euler equation, specifically the differential equation \(x^2 y'' + 8 x y' - 18 y = x^8\) with initial conditions \(y(1)=3\) and \(y'(1)=2\).

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • The original poster attempts to find a general solution and expresses confusion regarding the number of unknowns in their approach. Some participants suggest substituting the solution back into the differential equation to derive additional equations. Others propose changing variables or checking the associated homogeneous equation for insights.

Discussion Status

The discussion is active, with participants offering various approaches and suggestions for tackling the problem. There is no explicit consensus on a single method, but multiple lines of reasoning are being explored, including variable substitution and reference to related equations.

Contextual Notes

Participants note the nature of the equation as an "Euler type" and discuss the implications of this classification on the solution approach. There is mention of a potential extraneous solution and the need to verify results against the original equation.

lisa92
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Question:

Find y as a function of x:

x^2 y'' + 8 x y' - 18 y = x^8

y(1)=3, y'(1)=2


Attempted solution:

I found the general equation to be Ax^(-9)+Bx^2+Cx^8.
However when I try to solve the initial value problem for this equation I have 3 unknowns.
 
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Welcome to PF, lisa92! :smile:

Did you substitute your solution in the DE?
If you do, you'll find you have a 3rd equation.
 
write
(x2D2+8 x D-18)=(xD+9)(xD-2)
or change variables u=x2 y

your general solution is extraneous check it in the equation to eliminate
 


That's an "Euler type" equationl. The change of variable x= ln(t) changes it to a differential equation with constant coefficients which may be simpler.

If you do not wish to do it that way, what did you get as a solution to the associated homogeneous equation?
 


Check out Euler-Cauchy equations at the bottom of this link:

http://sosmath.com/tables/diffeq/diffeq.html

This will help you obtain the solution to the homogeneous equation.

The particular solution will probably be y = k * x^8 + y (homogeneous)
 

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