x=ln(1+t²)y=t-arctantdx/dt=2t/(1+t²)dy/dt=1-1/(1+t²)=t²/(1+t²)∴dy/dx=t/2∴d²y/dx²=½/[2t/(1+t²)]=(1+t²)/4t(y=t-arctanx,将x=ln(1+t²)代入，y=t-arctan[ln(1+t²)],计算过程一样）
