Giải thích các bước giải:
Bài 3:
$\lim \dfrac{4}{\sqrt{n+3}-\sqrt{n+1}}$
$=\lim \dfrac{4(\sqrt{n+3}+\sqrt{n+1})}{(\sqrt{n+3}-\sqrt{n+1})(\sqrt{n+3}+\sqrt{n+1})}$
$=\lim \dfrac{4(\sqrt{n+3}+\sqrt{n+1})}{n+3-(n+1)}$
$=\lim \dfrac{4(\sqrt{n+3}+\sqrt{n+1})}{2}$
$=\lim 2(\sqrt{n+3}+\sqrt{n+1})$
$=+\infty$