a^2+b^2+c^2=ab+bc+aca^2+b^2+c^2-ab-bc-ac=02a^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因为(a-b)^2>=0,(a-c)^2>=0,(b-c)^2>=0,要使(a-b)^2+(a-c)^2+(b-c)...