简单分析一下，答案如图所示
dx/dt=1-1/(1+t)=t/(1+t) dy/dt=3t^2+2t=t(3t+2) y'=dy/dx=(3t+2)(t+1)=3t^2+5t+2 y"=dy'/dx=(dy'/dt)/(dx/dt)=(6t+5)(1+t)/t=6t+11+5/t
