Theorem2. Let S , T arbitrary subsets of Zp, if|S| ≥ p+1 2 and|T| ≥ p+1 2 , then S + T = Zp. Proof. If c ∈ Zp, let W = −T + c = {−t + c : t ∈ T}, then |W| = |T| ≥ p+1 2 , therefore STW 6= ∅. Then there is s0 ∈ S and w0 ∈ W such that −t0 + c = s0 for some t0 ∈ T. Therefore c = s0 + t0 ∈ S + T