Giải thích các bước giải:
\(\begin{array}{l}
1,\\
\lim \left| {{u_n}} \right| = + \infty \Rightarrow \lim {u_n} = \pm \infty \\
\lim {u_n} = 0 \Rightarrow \lim \left| {{u_n}} \right| = 0\\
\lim {u_n} = - a \Rightarrow \left[ \begin{array}{l}
\lim \left| {{u_n}} \right| = - a\,\,\,\,\,\,\,\,\,\left( {a < 0} \right)\\
\lim \left| {{u_n}} \right| = a\,\,\,\,\,\,\,\,\,\,\,\,\left( {a > 0} \right)
\end{array} \right.\\
\Rightarrow C\\
4,\\
\lim \frac{{\sqrt {n + 1} - 4}}{{\sqrt {n + 1} + n}}\\
= \lim \dfrac{{\sqrt {1 + \frac{1}{n}} - \frac{4}{{\sqrt n }}}}{{\sqrt {1 + \frac{1}{n}} + \sqrt n }}\\
= 0\\
\left( \begin{array}{l}
\lim \left( {\sqrt {1 + \frac{1}{n}} - \frac{4}{{\sqrt n }}} \right) = \sqrt 1 - 0 = 1\\
\lim \left( {\sqrt {1 + \frac{1}{n}} + \sqrt n } \right) = + \infty
\end{array} \right)
\end{array}\)
