# Differential equation

1. Dec 7, 2004

### drdolittle

somebody slove this differential equations

1/y' = (1/y)+(1/x)

2. Dec 7, 2004

### Zurtex

Perhaps looking at it like this:

$$\frac{1}{\frac{dy}{dx}} = \frac{1}{y} + \frac{1}{x}$$

$$\frac{dx}{dy} = \frac{1}{y} + \frac{1}{x}$$

$$x\frac{dx}{dy} = \frac{x}{y} + 1$$

lol, I'll stop there because I suddenly realise this is beyond me (but it looks in a 'nicer' form, perhaps it will help you)

3. Dec 7, 2004

### drdolittle

Your solution is just a peanut compared to where i have gone....there is still more to go...anyhow thanx for trying,do try nmore and figure out the solution.

regards
drdolittle

4. Dec 7, 2004

### Zurtex

Well can you post what you have done please so others can help.

5. Dec 7, 2004

### Zurtex

I ran this through Mathematica: DSolve[1/(y'[x]) == 1/x + 1/y[x], y[x], x]

And it gave me nothing sorry.

Edit: Although I'm not used to using Mathematica and have yet to get it to solve the simplest thing I think I have inputed it right.

Last edited: Dec 7, 2004
6. Dec 7, 2004

### drdolittle

try seperation of variables...after that iam struggling to cotinue....

7. Dec 7, 2004

### daster

Even though I just started learning differential equations, I thought I'd give this a try:

$$\frac{dx}{dy}=\frac{1}{y}+\frac{1}{x}$$

$$\frac{dy}{dx}=\frac{xy}{x+y}$$

$$x\frac{dy}{dx}+y\frac{dy}{dx}=xy$$

$$y+x\frac{dy}{dx}+y\frac{dy}{dx}=y+xy$$

$$\frac{d(xy)}{dx}+\frac{1}{2}\frac{d(y^2)}{dx}=y(1+x)$$

$$\frac{1}{y}\,d(xy)+\frac{1}{2y}\,d(y^2)=(1+x)\,dx$$

I don't know what to do now, and I don't know if any of this is right, but I hope it'd be of some use.

8. Dec 8, 2004

### Zurtex

Err I still think this is beyond me but I think you made a mistake on the LHS going from the 4th to the 5th line as:

$$\frac{d(xy)}{dx} = x\frac{dy}{dx} + y$$

9. Dec 8, 2004

### daster

I added a y to the LHS in the 4th step.

10. Dec 8, 2004

### Integral

Staff Emeritus
What I see when I look at that equation is a family of hyperbolas very much like the simple lens equation. There is a change of variables and a rotation that will reduce this equation to something which may be separable. Unfortunately I do not have the time to do all of the algebra for you.

Explore doing a change of variables, perhaps to polar coordinates, see what you get.

11. Dec 13, 2004

### iSamer

How do you guys write the nice format of dy/dx and the fractions? Which program do you use, and you post them as photos?

I'll help in solving it, but after knowing how to post a math solution

12. Dec 13, 2004