证明:a*a+b*b+c*c-ab-ac-bc=a*a/2+b*b/2+c*c/2+a*a/2+b*b/2+c*c/2-2ab/2-2ac/2-2bc/2=1/2(a*a+b*b-2ab)+1/2(a*a+c*c-2ac)+1/2(b*b+c*c-2bc)=1/2(a-b)^2+1/2(a-c)^2+1/2(b-c)^2因为:(a-b)^2>=0;(a-c)^2>=0;(b-c)^2>=0...