a^2+b^2+c^2=ab+bc+ac
两边同乘以2,得到:
2a^2+2b^2+2c^2-2ab-2bc-2ac=0
a^2-2ab+b^2+a^2-2ac+c^2+b^2-2bc+c^2=0
(a-b)^2+(a-c)^2+(b-c)^2=0