y=x/(x+1)的二阶导数?y'=1/(x+1)²，y''=((x+1)^-2)'=-2(x+1)^-3=-2/(x+1)^3.
【速算】y=x/（x +1)=1-1/(x+1)y'=1/(x+1)^2y"=-2/(x+1)^3
看不懂啥意思
