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
所以,1/2(a-b)^2+1/2(a-c)^2+1/2(b-c)^2>=0
且仅当a=b=c时,a*a+b*b+c*c-ab-ac-bc=0
题目错了,或者你少写了