Đáp án:
$\begin{array}{l}
a)\sqrt {\left( {x + 1} \right)\left( {x - 1} \right)} \\
Dkxd:\left( {x + 1} \right)\left( {x - 1} \right) \ge 0\\
\Rightarrow \left[ \begin{array}{l}
\left\{ \begin{array}{l}
x + 1 \ge 0\\
x - 1 \ge 0
\end{array} \right.\\
\left\{ \begin{array}{l}
x + 1 \le 0\\
x - 1 \le 0
\end{array} \right.
\end{array} \right. \Rightarrow \left[ \begin{array}{l}
\left\{ \begin{array}{l}
x \ge - 1\\
x \ge 1
\end{array} \right.\\
\left\{ \begin{array}{l}
x \le - 1\\
x \le 1
\end{array} \right.
\end{array} \right. \Rightarrow \left[ \begin{array}{l}
x \ge 1\\
x \le - 1
\end{array} \right.\\
Vay\,x \ge 1\,hoac\,x \le - 1\\
b)\sqrt {2x} \\
Dkxd:2x \ge 0\\
\Rightarrow x \ge 0\\
Vay\,x \ge 0
\end{array}$
