2a²+2b²+2c²-2ab-2bc-2ac=(a²-2ab+b²)+(b²-2bc+c²)+(c²-2ac+a²)=(a-b)²+(b-c)²+(c-a)²平方大于等于0所以(a-b)²+(b-c)²+(c-a)²≥02a&su...