对方程两边对x求导,则：
y+xy'-y'e^y=0
--->y'+y'+xy''-y''e^y-(y')^2(e^y)=0
--->y'(2-y'e^y)=y''(e^y-x)
--->y''=y'(2-y'e^y)/(e^y-x)