update notations and fix typos

pages/Math4121/Math4121_L2.md

@@ -39,7 +39,7 @@ $$
 
 So $h'(x)=\frac{h(t)-h(x)}{t-x}=(f'(x)+u(t))(g'(y)+v(s))$. Since $u(t)\to 0$ and $v(s)\to 0$ as $t\to x$ and $s\to y$, we have $h'(x)=g'(y)f'(x)$.
 
-EOP
+QED
 
 #### Example 5.6
 
@@ -135,4 +135,4 @@ If $x<t<x+\delta$, then $f(x)\geq f(t)$ so $\frac{f(t)-f(x)}{t-x}\geq 0$.
 
 So $\lim_{t\to x}\frac{f(t)-f(x)}{t-x}=0$.
 
-EOP
+QED