3（a^2+b^2+c^2)=(a+b+c)^2
3（a^2+b^2+c^2)-(a+b+c)^2
=2a^2+2b^2+2c^2-2ab-2ac-2bc
=(a^2-2ab+b^)+(a^2-2ac+c^2)+(b^2-2bc+c^2)
=(a-b)^2+(a-c)^2+(b-c)^=0
则a-b=0,a-c=0,b-c=0
即a=b=c
2a-b-c=2a-a-a=0