In this article, we prove that the double inequality αP(a,b)+1(1-α)Q(a,b) < M(a,b) < βP(a,b)+(1-β)Q(a,b) holds for any a,b > 0 with a ≠ b if and only if α ≥ 1/2 and β ≤ [π(2log(1+2)-1)]/[(2π-2)log(1+2)]=03595. . ., where M(a,b), Q(a,b), and P(a,b) are the Neuman-Sándor, quadratic, and first Seiffert means of a and b, respectively. © 2014 Wuhan ...