(y-x)dy+ydx=0
ydy-xdy+ydx=0
(1/y)dy+(ydx-xdy)/y^2=0
d[lny+(x/y)]=0
积分,得通解为:ln︱y︱+(x/y)=C
代入初始条件:C=0
特解为: ln︱y︱+(x/y)=0