update notations and fix typos

pages/Math4121/Math4121_L5.md

@@ -34,7 +34,7 @@ Set $F(x)=-f(x)$. and $q=-A+\epsilon>-A$. Apply main step, $\exists c_2\in (a,b)
 
 We take $c=\min(c_1,c_2)$. Then $\forall x\in (a,c)$, $\frac{f(x)}{g(x)}<q$.
 
-EOP
+QED
 
 ### Higher Order Derivatives
 
@@ -114,6 +114,6 @@ By Mean Value Theorem, $\exists x_n\in (\alpha,x_{n-1})$ such that $g^{(n)}(x_n)
 
 Since $g^{(n)}(\alpha)=0$ for $k=0,1,2,\cdots,n-1$, we can find $x_n\in (\alpha,x_{n-1})$ such that $g^{(n)}(x_n)=0$.
 
-EOP
+QED
 
 ## Chapter 6: Riemann-Stieltjes Integral