Giải thích các bước giải:
\(
\begin{array}{l}
a)x^2 - 9 \ne 0 \Leftrightarrow x \ne \pm 3 \\
b)\left\{ {\begin{array}{*{20}c}
{x^2 - 4 \ne 0} \\
{3 - x \ge 0} \\
\end{array}} \right. \Leftrightarrow \left\{ {\begin{array}{*{20}c}
{x \ne \pm 2} \\
{x \le 3} \\
\end{array}} \right. \\
c)\left\{ {\begin{array}{*{20}c}
{3x + 1 > 0} \\
{3 - x \ge 0} \\
\end{array}} \right. \Leftrightarrow \left\{ {\begin{array}{*{20}c}
{x > \frac{{ - 1}}{3}} \\
{x \le 3} \\
\end{array}} \right. \Leftrightarrow \frac{{ - 1}}{3} < x \le 3 \\
d)\left\{ {\begin{array}{*{20}c}
{x - 1 \ne 0} \\
\begin{array}{l}
2x^2 - x - 1 \ne 0 \\
2x + 1 \ge 0 \\
\end{array} \\
\end{array}} \right. \Leftrightarrow \left\{ {\begin{array}{*{20}c}
{x \ne 1} \\
{x \ne 1} \\
\begin{array}{l}
x \ne \frac{{ - 1}}{2} \\
x \ge \frac{{ - 1}}{2} \\
\end{array} \\
\end{array}} \right. \Leftrightarrow \left\{ {\begin{array}{*{20}c}
{x > \frac{{ - 1}}{2}} \\
{x \ne 1} \\
\end{array}} \right. \\
\end{array}
\)
