y' = [2-e^(x+y)]/[1+e^(x+y)]=-1 + 3/[1+e^(x+y)]y''=3/梁纯棚裤档[1+e^(x+y)]=-3e^(x+y).(1+y')/[1+e^(x+y)]=-3e^(x+y).(1-1 + 3/[1+e^(x+y)])/橡则[1+e^(x+y)]^2=-9e^(x+y)/ [1+e^(x+y)]^3
