$\begin{array}{l}
a)\,\,\,\sqrt {2x + 5\,} \,\,\,xac\,\,dinh \Leftrightarrow 2x + 5 \ge 0 \Leftrightarrow x \ge - \frac{5}{2}.\\
b)\,\,\,\sqrt { - 2x - 5} \,\,\,\,xac\,\,dinh \Leftrightarrow - 2x - 5 \ge 0 \Leftrightarrow - 2x \ge 5 \Leftrightarrow x \le \frac{{ - 5}}{2}.\\
c)\,\,\,\sqrt {\frac{x}{{2x + 1}}} \,\,\,xac\,\,dinh\,\, \Leftrightarrow \frac{x}{{2x + 1}} \ge 0 \Leftrightarrow \left[ \begin{array}{l}
\left\{ \begin{array}{l}
x \ge 0\\
2x + 1 > 0
\end{array} \right.\\
\left\{ \begin{array}{l}
x \le 0\\
2x + 1 < 0
\end{array} \right.
\end{array} \right. \Leftrightarrow \left[ \begin{array}{l}
\left\{ \begin{array}{l}
x \ge 0\\
x > - \frac{1}{2}
\end{array} \right.\\
\left\{ \begin{array}{l}
x \le 0\\
x < - \frac{1}{2}
\end{array} \right.
\end{array} \right. \Leftrightarrow \left[ \begin{array}{l}
x \ge 0\\
x < - \frac{1}{2}
\end{array} \right..\\
d)\,\,\sqrt {\frac{7}{{{x^2}}}} \,\,\,xac\,\,dinh\,\, \Leftrightarrow {x^2} \ne 0 \Leftrightarrow x \ne 0.
\end{array}$
