（x·x+y.y）的完全平方-4xy（x·x+y·y）+4x·x·y·y
=（x·x+y.y）^2-2*2xy*（x·x+y·y）+(2x·y)^2
=（x·x+y.y-2x·y）^2
=[(x-y)^2]^2=[(-2/3)^2]^2=16/81