a²+b²+c²-ab-bc-ac=0
2(a²+b²+c²-ab-bc-ac)=0
2a²+2b²+2c²-2ab-2bc-2ac=0
(a²+b²-2ab）+（a²+c²-2ac）+（b²+c²-2bc）=0
(a-b)²+(a-c)²+(b-c)²=0
a-b=0||b-c=0||a-c=0
::a=b=c