(3)d(x-y)=d(x+y)/(x+y)=dln(x+y);x-y=ln(x+y)+C, P(x+y)=e^(x-y);(4)两边同时除以(x^2+y^2),有 dx+(ydx-xdy)/(x^2+y^2)=0;dx+d[arctan(x/y)] =0, x+arctan(x/y)=C.
