Hello,

I need to prove something related to 1st order exact equations. Here's the question:

Show that the diff. equation M+Ny'=0 has an integrating factor which is a function of z=x+y only if and only if

(dM/dy - dN/dx)/(M-N)

is a function of z only.

I'd just like to get a tip on how to start the problem. Thank you!