# Innocent looking 1st order DE

• May 9th 2009, 12:03 PM
milodino
Innocent looking 1st order DE
$2y e^{y/x} \frac{dy}{{dx}} - 2x - y = 0$

I'm not sure how difficult you guys at this forum might find this. It took me a long long long time to figure out (I am not a math major). Should need at least two substitutions.

(The solution, in implicit form, is $y^2 - x^2 e^{y/x} = C$).
• May 10th 2009, 03:16 PM
Jester
Quote:

Originally Posted by milodino
$2y e^{y/x} \frac{dy}{{dx}} - 2x - y = 0$

I'm not sure how difficult you guys at this forum might find this. It took me a long long long time to figure out (I am not a math major). Should need at least two substitutions.

(The solution, in implicit form, is $y^2 - x^2 e^{y/x} = C$).

If you divide your ODE by x then

$2\frac{y}{x} \,e^{\frac{y}{x}} \,\frac{dy}{{dx}} - 2 - \frac{y}{x} = 0$

which suggests the substitution $u = \frac{y}{x}$ and the ODE separates. Not really sure how you got your answer. An ODE that looks a lot like your and gives rise to your solution is

$
\left( 2y \,e^{-\frac{y}{x}} -x\right) y' - 2x + y = 0
$
• May 12th 2009, 01:35 AM
milodino
u = y/x is one of the substitutions, but it doesn't actually neatly separate after that.

$x \frac{du}{dx} + u = \frac{2+u}{2u e^u + 1}$

Admittedly it does reach a pretty nice form eventually but it looked pretty nasty here.