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Update 第二章_机器学习基础.md 

最小化类内方差，修正公式错误
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@ -856,7 +856,7 @@ u_j = \frac{1}{N_j} \sum_{\boldsymbol x\epsilon X_j}\boldsymbol x(j=0,1)，
$$
​ 假设投影直线是向量 $\boldsymbol w$，对任意样本 $\boldsymbol x_i$，它在直线 $w$上的投影为 $\boldsymbol w^Tx_i$，两个类别的中心点 $u_0$, $u_1 $在直线 $w$ 的投影分别为 $\boldsymbol w^Tu_0$ 、$\boldsymbol w^Tu_1$。
​ LDA的目标是让两类别的数据中心间的距离 $\| \boldsymbol w^Tu_0 - \boldsymbol w^Tu_1 \|^2_2$ 尽量大，与此同时，希望同类样本投影点的协方差$\boldsymbol w^T \sum_0 \boldsymbol w$、$\boldsymbol w^T \sum_1 \boldsymbol w$ 尽量小，最小化 $\boldsymbol w^T \sum_0 \boldsymbol w - \boldsymbol w^T \sum_1 \boldsymbol w​$ 。
​ LDA的目标是让两类别的数据中心间的距离 $\| \boldsymbol w^Tu_0 - \boldsymbol w^Tu_1 \|^2_2$ 尽量大，与此同时，希望同类样本投影点的协方差$\boldsymbol w^T \sum_0 \boldsymbol w$、$\boldsymbol w^T \sum_1 \boldsymbol w$ 尽量小，最小化 $\boldsymbol w^T \sum_0 \boldsymbol w + \boldsymbol w^T \sum_1 \boldsymbol w​$ 。
​ 定义
​ 类内散度矩阵
$$