Update Math4121_L4.md

pages/Math4121/Math4121_L4.md

@@ -90,12 +90,14 @@ Case 1: $f(x)\to 0$ and $g(x)\to 0$ as $x\to a$.
 
 As $x\to a$, $f(x)\to 0$ and $g(x)\to 0$. So
 
-$$\begin{aligned}
+$$
+\begin{aligned}
 \lim_{x\to a}\frac{f(x)-f(y)}{g(x)-g(y)}&=\lim_{x\to a}\frac{0-f(y)}{0-g(y)}\\
 &=\lim_{x\to a}\frac{f(y)}{g(y)}\\
 &=\frac{f'(y)}{g'(y)}\\
 &\leq r<q
-\end{aligned}$$
+\end{aligned}
+$$
 
 $\forall y\in (a,c)$, $\frac{f(y)}{g(y)}<q$.