\[x^{4} - 20x^{2} + 64 = 0\]
\[x^{2} = y \geq 0:\]
\[y^{2} - 20y + 64 = 0\]
\[D_{1} = 100 - 64 = 36\]
\[y_{1} = 10 + 6 = 16;\ \]
\[y_{2} = 10 - 6 = 4.\]
\[1)\ x^{2} = 16\]
\[x = \pm 4.\]
\[2)\ x^{2} = 4\]
\[x = \pm 2.\]
\[Ответ:x = \pm 2;x = \pm 4.\]