13. Show that if $a$ is a quadratic residue of the prime $p$, then the solutions of $x^{2} \equiv a(\bmod p)$ are
a) $x \equiv \pm a^{n+1}(\bmod p)$, if $p=4 n+3$.
b) $\quad x \equiv \pm 2^{2 n+1} a^{n+1}(\bmod p)$, if $p=8 n+5$