答：x=ln√(1+t^2),dx/dt=[1/√(1+t^2)]*(1/2)*2t/√(1+t^2)=t/(1+t^2)y=arctant,dy/dt=1/(1+t^2)所以：dy/dx=1/ty''=d²y/dx²=d(dy/dx)/dx=[d(1/t)/dt]/(dx/dt)=(-1/t^2)/[t/(1+t^2)]=-(1+t^2)/t^3所以：d...