\[x^{4} - 5x^{2} + 4 = 0\]
\[Пусть\ a = x^{2}:\]
\[a^{2} - 5a + 4 = 0\]
\[a_{1} + a_{2} = 5;\ \ \ a_{1} \cdot a_{2} = 4\]
\[a = 1;\ \ \ a = 4\]
\[a = 1:\]
\[x^{2} = 1\]
\[x = \pm 1.\]
\[a = 4:\]
\[x^{2} = 4\]
\[x = \pm 2\]
\[Ответ:числа\ \ \ \pm 1;\ \pm 2.\]