(a+b+c)^2=a^2+b^2+c^2+2ab+2bc+2aca^2+b^2≥2ab-----1/2(a^2+b^2)≥ab同理.1/2(b^2+c^2)≥bc 1/2(a^2+c^2)≥ac全加起来得: a^2+b^2+c^2≥ab+bc+ac把2ab+2bc+2ac加进去.(两边都加)(a+b+c)^2≥3ab+3ac+3bc即:(a+b+c)^2≥3(ab+bc+ac)
