因为:(a-b)²+(b-c)²+(a-c)²≥0
故有:a²-2ab+b²+b²-2bc+c²+a²-2ac+c²≥0
整理:2a²+2b²+2c²-(2ab+2bc+2ac )≥0
即:a²+b²+c²≥ab+bc+ac