update notations and fix typos

pages/Math4111/Math4111_L13.md

@@ -90,7 +90,7 @@ Thus, $\exists r>0$ such that $(w-r,w+r)\cap B=\phi$.
 
 Let $z=w+\frac{r}{2}$, then $x,y,z$ satisfy the desired properties.
 
-EOP
+QED
 
 ## Chapter 3: Numerical Sequences and Series
 
@@ -140,4 +140,4 @@ Let $n\geq N$ (arbitrary)
 
 Then $|s_n-q|=\frac{1}{n}\leq \frac{1}{N}\leq \epsilon$
 
-EOP
+QED