# Differential Equation Problem

• January 27th 2008, 12:55 PM
jblorien
Differential Equation Problem
(x+y) * (dy/dx) = (x-y)

the given answer is y^2-2xy-x^2 = C

The problem is in a section of the textbook where substitution methods are taught. That is, making a subsitution where v = some term containing the variables x and y. Then solving the substitution equation so that y = some term containing the variables x and v. Taking the derivative of that term so dy/dx can be substituted for. Then the next step would be to separate the variables x and v. integrate, and lastly you substutite x and y back in for v to gain your solution to the Diff EQN.

Problem is, I can't do it.

Help is much appreciated!

JB
• January 27th 2008, 01:02 PM
Krizalid
Quote:

Originally Posted by jblorien
(x+y) * (dy/dx) = (x-y)

the given answer is y^2-2xy-x^2 = C

First rewrite your equation as follows:

$\frac{{dy}}
{{dx}} = \frac{{x - y}}
{{x + y}}.$

Now multiply top & bottom by $\frac1x,$

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

Now it remains to substitute $y=ux.$
• January 27th 2008, 01:06 PM
jblorien
Thank you