a^2+b^2+c^2-ab-bc-ca=0两端乘22a^2+2b^2+2c^2-2ab-2bc-2ca=0a^2 - 2ab + b^2 + b^2 -2bc + c^2 + c^2 -2ac + a^2 = 0(a-b)^2 + (b-c)^2 + (c-a)^2 = 0a-b = 0b-c = 0c-a = 0所以 a = b =c