中文说明：BP神经网络的非线性系统建模--非线性函数的拟合:通过一个简单是二元二次函数，实现bps神经网络的非线性拟合，逼近真实值。
English Description：
BP neural network for nonlinear systems modeling non-linear function fitting: through the simple bivariate quadratic function, bps nonlinear fitting can be realized, approach the true values.