y=ln(1+x+y)
dy/dx=1/(1+x+y)*(0+1+dy/dx)
=1/(1+x+y) +1/(1+x+y)*dy/dx
dy/dx - 1/(1+x+y)*dy/dx =1/(1+x+y)
(1+x+y-1)/(1+x+y)dy/dx=1/(1+x+y)
dy/dx=1/(x+y)