a∧2 + b∧2 + c∧2 - ab - bc - ac = 0
2*a∧2 + 2*b∧2 + 2*c∧2 - 2ab - 2bc - 2ac = 0
(a-b)^2+(b-c)^2+(c-a)^2=0
所以a = b = c