Solution
= Solution
{c}
Again $s=(a/p)\in\{1,-1\}$. By <Euler's criterion>, $(s/p)=s^{(p-1)/2}$. If $p\equiv1\pmod4$, the exponent is even and the answer is one. If $p\equiv3\pmod4$, it is odd and the answer is $s$. Hence
$$
\boxed{\left(\frac{(a/p)}p\right)=\begin{cases}1,&p\equiv1\pmod4,\\(a/p),&p\equiv3\pmod4.\end{cases}}
$$