设 f(x) = arctanx - x + (x³/3),f(0) = 0f '(x) = 1/(1+x²) - 1 + x² = x^4 /(1+x²) > 0在[0,x] 上应用Lagrange中值定理,f(x) - f(0) = f '(ξ) * x > 0 （x >0）即证 arctanx > x - (x^3/3) (...