a(1)=s(1)=k,a(n+1)=s(n+1)-s(n)=k(2n+1)=2nk + k,a(n) = 2(n-1)k + k,0 < a(n+1) - a(n) = 2nk + k - [2(n-1)k + k] = 2k,k > 0.