y = x + ln y两边求导：y'=1+y'/yy'(1-1/y)=1y'=y/(y-1)y‘’=[y'(y-1)-y*y']/(y-1)² =[y-y²/(y-1)](y-1)² =[y(y-1)-y²]/(y-1)³ =-y/(y-1)³