2a²+2b²+2c²=2ab+2bc+2ac
化简：a^2-2ab+b^2+a^2-2ac+c^2+b^2-2bc+c^2=0
(a-b)^2+(a-c)^2+(b-a)^2=0
a=b
b=c,
a=c
是等边三角形