\[x^{4} - 13x^{2} + 36 = 0\]
\[Пусть\ x^{2} = y:\]
\[y^{2} - 13y + 36 = 0\]
\[y_{1} + y_{2} = 13;\ \ \ y_{1} \cdot y_{2} = 36\]
\[y_{1} = 9;\ \ \ y_{2} = 4.\]
\[Подставим:\]
\[1)\ x^{2} = 9\]
\[x = \pm 3.\]
\[2)\ x^{2} = 4\]
\[x = \pm 2.\]
\[Ответ:числа \pm 2;\ \pm 3.\]