选ca*a+b*b+c*c=ab+bc+ac2(a*a+b*b+c*c)=2(ab+bc+ac)2a*a+2b*b+2c*c=2ab+2bc+2ac(a*a+b*b-2ab)+(b*b+c*c-2bc)+(a*a+c*c-2ac)=(2ab+2bc+2ac)-(2ab+2bc+2ac)(a-b)（a-b）+(b-c)(b-c)+(a-c)(a-c)=0so a=b=c