8. 求 $x^{4}+y^{4} \equiv 1(\bmod 5)$ 的全部解 $\{x, y\}$.
8. $\begin{array}{ll}\{0 \bmod 5, \pm 1 \bmod 5\}, & \{0 \bmod 5, \pm 2 \bmod 5\} \\ \{ \pm 1 \bmod 5,0 \bmod 5\}, & \{ \pm 2 \bmod 5,0 \bmod 5\} .\end{array}$