一阶导数的常用差分逼近 求解df的差分逼近 flag标记使用的差分算法 算法来自周超红的《一阶导数的差分逼近汇总》 可按照里边的算法构造更高阶的差分逼近-First derivative of the common difference approximation approach for solving differential df flag tag algorithm using the difference method from the week of super