两边同时乘以2
则2a^2+b^2+2c^2-2ab-2bc-2ac=0
(a-b)^2+(b-c)^2+(a-c)^2=0
由于(a-b)^2≥0 (b-c)^2≥0 (a-c)^2≥0
a-b=0 b-c=0 a-c=0
a=b=c