# Basic differential equation

• July 22nd 2009, 03:10 AM
Basic differential equation
I am stumped by the following problem:

dy/dx = x+y, I am suppose to solve using the substitution: u=x+y

I'm really confused by this one, I don't even know where to start. Anyone?
• July 22nd 2009, 04:02 AM
Jester
Quote:

$u = x + y$ so $\frac{du}{dx} = \frac{dy}{dx} + 1$ so your DE becomes $\frac{du}{dx} = u + 1$ which is separable.