证明:∵四边形ABCD是平行四边形. ∴AB=CD;AB∥CD. ∴⊿AEO∽⊿CDO,AO/OC=EO/OD=AE/CD. ∵AE=AB/2=CD/2. ∴AO/OC=EO/OD=AE/CD=(CD/2)/CD=1/2. 故AO/AC=EO/ED=1/3,即点O分别是ED与AC的三等分点.
