How to find an integrating factor for this problem?

  • Thread starter AdrianZ
  • Start date
  • #1
319
0

Homework Statement


Find an integrating factor for:
xdy - (y + x^2 + 9y^2)dx = 0


Homework Equations


P(x,y)dx + Q(x,y)dy = 0
Δμ = μyP - μxQ.
where μ is the integrating factor.

The Attempt at a Solution


well, I don't know what I should do. I can use the formula I wrote but that would lead to solving a PDE. any ideas?
 

Answers and Replies

  • #2
319
0
No ideas?
 
  • #3
121
0
It would take a pretty lucky guess to come up with an integrating factor directly. However, you can use a Riccati transform and then back-track to find an integrating factor if that's your pleasure.

If you substitute y(x) = - x (dw/dx) / (9 w) where w is a function of x, you'll wind up with a linear 2nd order ODE in w.

Wikipedia has a decent description: http://en.wikipedia.org/wiki/Riccati_equation
 
  • #4
319
0
I don't know how to solve a linear 2nd order ODE yet. I mean I know but our professor hasn't started the second chapter which is about 2nd order linear equations. so I can't transform it into a 2nd order linear ODE. Is there a way to solve it without transforming it into a 2nd order linear ODE by finding an integrating factor?
 
  • #5
121
0
I sure don't see an obvious way. Is it possible that the equation should be

y dy - (y + x^2 + 9 y^2) dy = 0 ?

An integrating factor would be simple to find in that case.
 
  • #6
319
0
I sure don't see an obvious way. Is it possible that the equation should be

y dy - (y + x^2 + 9 y^2) dy = 0 ?

An integrating factor would be simple to find in that case.
No :(
 
  • #7
lurflurf
Homework Helper
2,440
138
group in terms of 3y/x to make the integrating factor easy to see
x dy - (y + x^2 + 9y^2)dx = 0
(x2/3)d(3y/x)-x2((3y/x)2+1)dx=0
 
  • #8
319
0
group in terms of 3y/x to make the integrating factor easy to see
x dy - (y + x^2 + 9y^2)dx = 0
(x2/3)d(3y/x)-x2((3y/x)2+1)dx=0
Now It's clear what I should do next. but I don't understand how you obtained the second equation from the first that directly.
 
  • #9
lurflurf
Homework Helper
2,440
138
It helps that I knew that
x dy-y dx=x2d(y/x)
from there it was clear x and y/x would be better variables that x and y.
These take a bit of practice.
There are a few others, but I keep in mind these common examples
d(xy)=x dy+y dx
y2 d(x/y)=y dx-x dy
x2 d(y/x)=x dy-y dx
(x2+y2) d(arctan(y/x))=x dy-y dx
[tex] x^{1-p}y^{1-q}{d}(x^py^q)=pydx+qxdy[/tex]
 

Related Threads on How to find an integrating factor for this problem?

  • Last Post
Replies
24
Views
1K
Replies
6
Views
799
  • Last Post
Replies
2
Views
1K
  • Last Post
Replies
3
Views
571
Replies
1
Views
2K
  • Last Post
Replies
4
Views
2K
Replies
2
Views
1K
Replies
4
Views
1K
  • Last Post
Replies
1
Views
1K
Replies
8
Views
6K
Top