a^2+b^2+c^2+d^2=2(ac+bd)
a^2-2ac+c^2+b^2-2bd+d^2=0
(a-c)^2+(b-d)^2=0
(a-c)^2=0 (b-d)^2=0
a-c=0 b-d=0
a=c b=d
得证四边形是平行四边形