\[x^{4} - 50x^{2} + 49 = 0\]
\[Пусть\ t = x^{2} \geq 0:\]
\[t^{2} - 50t + 49 = 0\]
\[t_{1} + t_{2} = 50;\ \ t_{1} \cdot t_{2} = 49\]
\[\Longrightarrow t_{1} = 49,\ \ t_{2} = 1.\]
\[x^{2} = 49\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ x² = 1\]
\[x = \pm 7\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ x = \pm 1\]
\[Ответ:\ x = \pm 7;\ x = \pm 1.\]