x＝t - arctant，y＝ln(1＋t²)，则 dy/dx ＝ (dy/dt) / (dx/dt)＝[2t/(1＋t²)] / [1 - 1/(1＋t²)]＝2 / t，d²y/dx² ＝ d(dy/dx) / dx＝(-2/t²) / [1 - 1/(1＋t²)]＝-2(1＋t²) / t^4
