解析函数的四则运算可以放心大胆的去做.z → 0时.分子z-zcos(z) = z-z(1-z²/2+o(z²)) = z³/2+o(z³) = z³(1/2+o(1)).分母z-sin(z) = z-(z-z³/6+o(z³)) = z³/6+o(z³) = ...