y-1=xe^y
两边同时对x求导得
y'=e^y+xe^y*y'
(1-xe^y)y'=e^y
y'=e^y/(1-xe^y)
=e^y/(2-y)
y''=(e^y*y'+e^y*y')/(2-y)²
=(2e^y)e^y/(2-y)³
=2e^2y/(2-y)³