Giải thích các bước giải:
Ta có:
\(\begin{array}{l}
1,\\
\left( {5 - x} \right)\left( {8 - x} \right) < 0\\
\Leftrightarrow \left[ \begin{array}{l}
\left\{ \begin{array}{l}
5 - x > 0\\
8 - x < 0
\end{array} \right.\\
\left\{ \begin{array}{l}
5 - x < 0\\
8 - x > 0
\end{array} \right.
\end{array} \right. \Leftrightarrow \left[ \begin{array}{l}
\left\{ \begin{array}{l}
x < 5\\
x > 8
\end{array} \right.\\
\left\{ \begin{array}{l}
x > 5\\
x < 8
\end{array} \right.
\end{array} \right. \Leftrightarrow 5 < x < 8\\
2,\\
1 < \left| x \right| < 3\\
\Leftrightarrow \left\{ \begin{array}{l}
\left| x \right| > 1\\
\left| x \right| < 3
\end{array} \right. \Leftrightarrow \left\{ \begin{array}{l}
\left[ \begin{array}{l}
x > 1\\
x < - 1
\end{array} \right.\\
- 3 < x < 3
\end{array} \right. \Leftrightarrow \left[ \begin{array}{l}
- 3 < x < - 1\\
1 < x < 3
\end{array} \right.\\
3,\\
\left| { - x} \right| = 3,6\\
\Leftrightarrow \left[ \begin{array}{l}
- x = 3,6\\
- x = - 3,6
\end{array} \right. \Leftrightarrow \left[ \begin{array}{l}
x = - 3,6\\
x = 3,6
\end{array} \right.
\end{array}\)
