a1,a2,a3≥0a_1, a_2, a_3 \ge 0a1,a2,a3≥0
求证a1+a2+a3+3a1a2a33≥2(a1a2+a2a3+a1a3)a_1 + a_2 + a_3 + 3\sqrt[3]{a_1 a_2 a_3} \ge 2(\sqrt{a_1 a_2}+\sqrt{a_2 a_3} +\sqrt{a_1 a_3})a1+a2+a3+33a1a2a3≥2(a1a2+a2a3+a1a3)
并确定等号成立的条件