说明： 该函数用来计算任意函数的一阶偏导数(数值方法),三相光伏逆变并网的仿真，音频信号通过LM386放大。 ( This function is used to calculate the arbitrary function of the first order partial derivative (numerical methods), Three-phase photovoltaic inverter and network simulation, LM386 audio signal amplification.)
