均值便宜跟踪的示例，利用最小二乘法进行拟合多元非线性方程，该函数用来计算任意函数的一阶偏导数(数值方法)。- Example tracking mean cheap, Multivariate least squares fitting method of nonlinear equations, This function is used to calculate the arbitrary function of the first order partial derivative (numerical methods).