y'=1/(x+y)^2(x+y)'-1=1/(x+y)^2t=x+yt'-1=1/t^2t'=1+1/t^2(t^2/(1+t^2))dt=dxt-atant=x+cx+y-atan(x+y)=x+cy=atan(x+y)+c
这是求什么？
