Can we use y=vx for non-homogeneous differential equation?

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Homework Help Overview

The discussion revolves around the use of the substitution \( y = vx \) in the context of non-homogeneous differential equations. The original poster presents an example equation, \( yy' = x^3 + \frac{y^2}{x} \), and seeks to understand the applicability of this substitution method.

Discussion Character

  • Exploratory, Assumption checking

Approaches and Questions Raised

  • Participants explore the substitution \( y = vx \) and its implications for solving the given non-homogeneous differential equation. There is an attempt to manipulate the equation through substitution and integration, leading to a potential solution form. Some participants question the validity of this approach for non-homogeneous cases and seek clarification on alternative solutions.

Discussion Status

The discussion is ongoing, with participants sharing their attempts at manipulating the equation and expressing curiosity about alternative methods. A hint for another solution has been requested, indicating an active exploration of different approaches.

Contextual Notes

The original poster's example is noted as not homogeneous, which raises questions about the appropriateness of the substitution method in this context. There is also a reference to the characteristics of homogeneous differential equations in the discussion.

jasoncurious
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Homework Statement


Can we use y=vx for non-homogeneous differential equation?

Example:
yy'=x^3+(y^2/x)→not homogeneous


Homework Equations


y=vx
dy/dx=v+x(dv/dx)

The Attempt at a Solution


By substituting the equation above:
vx(v+x dv/dx)=x^3+(v^2 x^2)/x
v^2*x+vx^2 dv/dx=x^3+v^2*x
Eliminate the v^2*x:
vx^2 dv/dx=x^3
Divide both sides with x^2:
v dv/dx=x
vdv=xdx
Continue the integration:
y^2=x^2(x^2+c), where c is a constant
 
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The substitution y=vx makes it easier to find the solution, because a homogenous differential equation takes the form:

##\frac{dy}{dx}=\frac{f_1(x,y)}{f_2(x,y)}##

which then equals ##\frac{f(y/x)}{g(y/x)}## or ##\frac{f(x/y)}{g(x/y)}## by taking ##x^n## or ##y^n## common, if f1 and f2 are homogenous in degree n.
 
jasoncurious said:

Homework Statement


Can we use y=vx for non-homogeneous differential equation?

Example:
yy'=x^3+(y^2/x)→not homogeneous


Homework Equations


y=vx
dy/dx=v+x(dv/dx)

The Attempt at a Solution


By substituting the equation above:
vx(v+x dv/dx)=x^3+(v^2 x^2)/x
v^2*x+vx^2 dv/dx=x^3+v^2*x
Eliminate the v^2*x:
vx^2 dv/dx=x^3
Divide both sides with x^2:
v dv/dx=x
vdv=xdx
Continue the integration:
y^2=x^2(x^2+c), where c is a constant

Easier: ##v \, dv/dx = d(v^2/2)/dx.## And, there are two solutions.
 
May I know the hint to the other solution? Thanks
 

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