ODE: Confused about a Homogeneous Eqn question

Click For Summary
SUMMARY

The discussion centers on solving a homogeneous differential equation represented by dy/dx = (x^2 + xy + y^2) / x^2. The participants confirm its homogeneity by expressing the right-hand side solely in terms of y/x. The transformation using v = y/x leads to the equation dx/x = dv/(v^2 + v + 1). However, discrepancies arise when integrating, as the expected solution involves arctan(y/x) - ln|x| = c, contrasting with the derived terms that include an additional v component.

PREREQUISITES
  • Understanding of homogeneous differential equations
  • Familiarity with substitution methods in differential equations
  • Knowledge of integration techniques involving arctan functions
  • Proficiency in manipulating algebraic expressions involving variables
NEXT STEPS
  • Study the properties of homogeneous equations in differential calculus
  • Learn about the method of substitution in solving differential equations
  • Explore integration techniques for functions involving arctan
  • Review examples of transforming variables in differential equations
USEFUL FOR

Students and educators in mathematics, particularly those focused on differential equations, as well as anyone seeking to deepen their understanding of homogeneous equations and integration techniques.

hahutzy
Messages
10
Reaction score
0

Homework Statement



So the problem goes like this:

Code:
dy/dx = ( x^2 + xy + y^2 ) / x^2
a) Show that it is a homogeneous equation.
b) Let v = y/x and express the eqn in x and v
c) Solve for y

Homework Equations



(Included)

The Attempt at a Solution



Code:
a) dy/dx = ( x^2 + xy + y^2 ) / x^2 * [(1/xy) / (1/xy)]
         = [(x/y) + 1 + (y/x)] / (x/y)
Since RHS is expressed only in terms of y/x, therefore it is homogeneous.

b) v = y/x
   y = vx
   dy/dv = x
   dy/dx = (dy/dv) (dv/dx) = x (dv/dx)
.'.dy/dx = [(x/y) + 1 + (y/x)] / (x/y) becomes
   x (dv/dx) = [(1/v) + 1 + v] / (1/v)
             = v^2 + v + 1
   dx/x = dv/(v^2 + v + 1)

The problem is, when I solve by integrating both sides, I get some gibberish arctan (2v/sqrt(3) + 1)term.
The solution in the book and solved on wolfram alpha is:
arctan (y/x) - ln |x| = c

However, this answer suggests that the equation in b) must've been
dx/x = dv/(v^2 + 1) <- no v term

Which is clearly not the case...

What am I doing wrong??
 
Physics news on Phys.org
\frac {dy}{dx} = x \frac {dv}{dx} + v
 
Random Variable said:
\frac {dy}{dx} = x \frac {dv}{dx} + v

Thank you!
 

Similar threads

  • · Replies 2 ·
Replies
2
Views
2K
  • · Replies 11 ·
Replies
11
Views
1K
  • · Replies 5 ·
Replies
5
Views
1K
Replies
21
Views
2K
Replies
3
Views
2K
Replies
4
Views
2K
  • · Replies 25 ·
Replies
25
Views
2K
Replies
2
Views
1K
Replies
2
Views
1K
Replies
4
Views
2K