\[x^{4} - 16x^{2} = 0\]
\[x^{2}\left( x^{2} - 16 \right) = 0\]
\[1)\ x^{2} = 0\]
\[x = 0.\]
\[2)\ x^{2} - 16 = 0\]
\[x^{2} = 16\]
\[x = \pm 4.\]
\[Ответ:x = 0;x = \pm 4.\]