Solution
= Solution
The <first supplementary law for quadratic reciprocity> is
$$
\boxed{(-1/p)=(-1)^{(p-1)/2}=\begin{cases}1,&p\equiv1\pmod4,\\-1,&p\equiv3\pmod4.\end{cases}}
$$
It also follows immediately from <Euler's criterion> applied to $-1$.