Question involving the solution to a Lagrange Differential Equation

In summary, the conversation is about solving a first order linear differential equation in standard form. The suggested method leads to an integral involving f'(P)dp/(P-f(P)), which can be simplified by using the substitution u(p)=p-f(p). However, the first term in the integral cannot be simplified any further without knowing the value of f. The conversation also mentions a website that discusses solving these types of equations and the person's curiosity about the solutions.
  • #1
jbowers9
89
1

Homework Statement



y = xf(y') + g(y')

Let y' = P
taking d/dx and rearranging gives

dx/dP - xf'(P)/{P - f(P)} = g'(P)/(P - f(P))

a 1st order linear differential equation in standard form.

Homework Equations



When I attempt to solve by the suggested standard method, I end up with the following integral:

[tex]\int f'(P)dp/(P - f(P))[/tex]

The Attempt at a Solution


I'm at a loss as how to go about integrating it.
 
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  • #2
Try the substitution [itex]u(p)=p-f(p)[/itex]
 
  • #3
I did and I got

[tex]\int dP/u(P) - ln|u(P)|[/tex]
What do I do with the 1st term?
 
  • #4
I don't think you can simplify it any further without knowing what [itex]f[/itex] is. At least your integral no longer involves [itex]f'[/itex] though.
 
  • #5
P = y' ; y' = dy/dx I believe.

The following site lays it out: http://www.newcircuits.com/articles.php
under Lagrange differential equation.

I was lead to this site after setting up the Euler-Lagrange equations in r,phi,theta, and became curious as to how to solve them or what kind of solutions they have. ie exact, series. Thanks.
 

FAQ: Question involving the solution to a Lagrange Differential Equation

What is a Lagrange Differential Equation?

A Lagrange Differential Equation is a type of second-order differential equation that involves a function and its derivatives. It is used to model various physical systems and is named after the mathematician Joseph-Louis Lagrange.

How is a Lagrange Differential Equation solved?

A Lagrange Differential Equation can be solved using various methods such as separation of variables, substitution, or using a power series. The specific method used depends on the form of the equation and the initial conditions given.

What are the applications of Lagrange Differential Equations?

Lagrange Differential Equations have various applications in physics, engineering, and other fields. They are commonly used to model systems such as pendulums, electrical circuits, and planetary motion.

Can Lagrange Differential Equations have multiple solutions?

Yes, Lagrange Differential Equations can have multiple solutions. This is because they are second-order differential equations and therefore have two arbitrary constants that can result in different solutions.

Are there any real-world limitations to Lagrange Differential Equations?

While Lagrange Differential Equations are useful for modeling many physical systems, they do have some limitations. They may not accurately represent systems with highly non-linear behavior or those with varying parameters. In addition, they may not be able to predict the behavior of a system beyond a certain point in time.

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