diff --git a/markdown/week8.md b/markdown/week8.md
index e71b4cad..3a1dbfd7 100644
--- a/markdown/week8.md
+++ b/markdown/week8.md
@@ -150,6 +150,7 @@ $J(A,B)=\frac{\left| A\cap B \right|}{\left|A\cup B \right|}$
 $n$维向量$x$和$y$的夹角记做$\theta$，根据余弦定理，其余弦值为：
 
 $cos (\theta )=\frac{{{x}^{T}}y}{\left|x \right|\cdot \left| y \right|}=\frac{\sum\limits_{i=1}^{n}{{{x}_{i}}{{y}_{i}}}}{\sqrt{\sum\limits_{i=1}^{n}{{{x}_{i}}^{2}}}\sqrt{\sum\limits_{i=1}^{n}{{{y}_{i}}^{2}}}}$
+
 (4). Pearson皮尔逊相关系数：
 ${{\rho }_{XY}}=\frac{\operatorname{cov}(X,Y)}{{{\sigma }_{X}}{{\sigma }_{Y}}}=\frac{E[(X-{{\mu }_{X}})(Y-{{\mu }_{Y}})]}{{{\sigma }_{X}}{{\sigma }_{Y}}}=\frac{\sum\limits_{i=1}^{n}{(x-{{\mu }_{X}})(y-{{\mu }_{Y}})}}{\sqrt{\sum\limits_{i=1}^{n}{{{(x-{{\mu }_{X}})}^{2}}}}\sqrt{\sum\limits_{i=1}^{n}{{{(y-{{\mu }_{Y}})}^{2}}}}}$