利用最小二乘法来对w和b进行估计。最小二乘法就是试图找到一条直线，使得所有样本到直线上的欧式距离之和最小。(The least squares method is used to estimate W and B. The least squares method is trying to find a straight line, so that the sum of Euclidean distance of all the samples to the line is minimum)