Given that p and q are odd primes. Suppose we know whether q is a quadratic residue of p or not. The question that this section will answer is whether p will be a quadratic residue of q or...Given that p and q are odd primes. Suppose we know whether q is a quadratic residue of p or not. The question that this section will answer is whether p will be a quadratic residue of q or not. Before we state the law of quadratic reciprocity, we will present a Lemma of Eisenstein which will be used in the proof of the law of reciprocity. The following lemma will relate Legendre symbol to the counting lattice points in the triangle.