(a^2+b^2)(c^2 + d^2)≥(ac+bd)^2;
等号成立条件：ad=bc
扩展:（(a1)^2+(a2)^2+(a3)^2+...+(an)^2)((b1)^2+(b2)^2+(b3)^2+...(bn)^2)≥(a1b1+a2b2+a3b3+..+anbn)^2;
等号成立条件：a1:b1=a2:b2=…=an:bn