\[\begin{array}{*{20}{l}}
\begin{array}{l}
1,{\left( {2X - 5} \right)^2} - {\left( {3 - X} \right)^2} = 0\\
\Leftrightarrow \left( {2x - 5 - 3 + x} \right)\left( {2x - 5 + 3 - x} \right) = 0\\
\Leftrightarrow \left( {3x - 8} \right)\left( {x - 2} \right) = 0\\
\Leftrightarrow \left[ \begin{array}{l}
3x - 8 = 0\\
x - 2 = 0
\end{array} \right.\\
\Leftrightarrow \left[ \begin{array}{l}
x = \frac{8}{3}\\
x = 2
\end{array} \right.\\
vay:S = \{ \frac{8}{3};2\}
\end{array}\\
\begin{array}{l}
2.5{X^3} - 45x = 0\\
\Leftrightarrow 5x\left( {{x^2} - 9} \right) = 0\\
\Leftrightarrow 5x\left( {x - 3} \right)\left( {x + 3} \right) = 0\\
\Leftrightarrow \left[ \begin{array}{l}
5x = 0\\
x - 3 = 0\\
x + 3 = 0
\end{array} \right.\\
\Leftrightarrow \left[ \begin{array}{l}
x = 0\\
x = 3\\
x = - 3
\end{array} \right.\\
vay:S = \{ 0;3; - 3\}
\end{array}\\
\begin{array}{l}
3,5\left( {x - 3} \right) = {x^2}{\rm{ }} - 6x + 9\\
\Leftrightarrow 5\left( {x - 3} \right) = {\left( {x - 3} \right)^2}\\
\Leftrightarrow 5\left( {x - 3} \right) - {\left( {x - 3} \right)^2} = 0\\
\Leftrightarrow \left( {x - 3} \right)\left( {5 - x + 3} \right) = 0\\
\Leftrightarrow \left( {x - 3} \right)\left( {8 - x} \right) = 0\\
\Leftrightarrow \left[ \begin{array}{l}
x - 3 = 0\\
8 - x = 0
\end{array} \right.\\
\Leftrightarrow \left[ \begin{array}{l}
x = 3\\
x = 8
\end{array} \right.\\
vay:S = \{ 3;8\}
\end{array}\\
\begin{array}{l}
4,\left( {{x^2} - 9} \right)\left( {{x^2} + x + 1} \right) = 0\\
\Leftrightarrow {x^2} - 9 = 0\left( {do:{x^2} + x + 1 > 0} \right)\\
\Leftrightarrow {x^2} = 9\\
\Leftrightarrow x = \pm 3\\
vay:S = \{ - 3;3\}
\end{array}
\end{array}\]