\[Уравнение\ не\ имеет\ корней,\ \]
\[если\ D < 0.\]
\[x^{4} - 8x^{2} + a = 0\]
\[Пусть\ x^{2} = y \geq 0:\]
\[y^{2} - 8y + a = 0\]
\[D = 16 - a\]
\[16 - a < 0\]
\[- a < - 16\]
\[a > 16.\]
\[Ответ:при\ a > 16.\]