Xét:

\[\begin{array}{l}
\left( {{x^4} - 8{x^2} - 9} \right)\left( {{x^2} - 16} \right) = 0\\
\Leftrightarrow \left( {{x^4} + {x^2} - 9{x^2} - 9} \right)\left( {x - 4} \right)\left( {x + 4} \right) = 0\\
\Leftrightarrow \left[ {{x^2}\left( {{x^2} + 1} \right) - 9\left( {{x^2} + 1} \right)} \right]\left( {x - 4} \right)\left( {x + 4} \right) = 0\\
\Leftrightarrow \left( {{x^2} + 1} \right)\left( {{x^2} - 9} \right)\left( {x - 4} \right)\left( {x + 4} \right) = 0\\
\Leftrightarrow \left( {{x^2} + 1} \right)\left( {x - 3} \right)\left( {x + 3} \right)\left( {x - 4} \right)\left( {x + 4} \right) = 0\\
\Leftrightarrow \left[ \begin{array}{l}
{x^2} + 1 = 0\\
x - 3 = 0\\
x + 3 = 0\\
x - 4 = 0\\
x + 4 = 0
\end{array} \right.\\
\Leftrightarrow \left[ \begin{array}{l}
x = 3\\
x = - 3\\
x = 4\\
x = - 4
\end{array} \right.\\
vay:A = \{ - 4; - 3;3;4\}
\end{array}\]