BE=AE-AB=2AC/3-ABCD=AD-AC=AB/2-AC故：BE·CD=(2AC/3-AB)·(AB/2-AC)=-|AB|^2/2-2|AC|^2/3+4AB·AC/3=-1/2-2/3+(4/3)cosA=-1/2-2/3+2/3=-1/2
