a²+b²+c²-ab-bc-ac=[2a²+2b²+2c²-2ab-2bc-2ac]/2=[(a²-2ab+b²)+(b²-2bc+c²)+(c²-2ac+a²)]/2=[(a-b)²+(b-c)²+(c-a)²]/2∵平方大于等于0∴[(a-b)²+(b-c)²+(c-a)²]/2≥0∴a²+b²+c²-ab-bc-ac≥0
