# Cant solve diff-eq with substitution

1. Jan 17, 2005

### Ryoukomaru

Use subs $$y=xv$$ to show that $$(x^2+y^2)+2xy\frac{dy}{dx}=0, x>0$$ is $$x^3+3xy^2=k$$ where k is a constant.

I played around with this at school and if memory serves me correct i got something similar to $$\frac{dx}{dv}=\frac{-3}{2xv}-\frac{1}{2}$$ and after that i decided i wasnt on the right path and stopped. Need a little help here !

2. Jan 17, 2005

### arildno

x is to remain the independent variable; v(x) is the dependent variable which replaces y(x)

3. Jan 17, 2005

### stunner5000pt

What is v? Is v a function or a constant??

4. Jan 17, 2005

### arildno

v is defined as the function:
$$v(x)=\frac{y(x)}{x}$$

5. Jan 17, 2005

### HallsofIvy

Staff Emeritus
If y= xv, then dy/dx= x dv/dx+ v. Put that into your equation and replace y by xv and see what happens.

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