六边形好证,五边形则不好证明:
O是中心,则:OA1=-OA4
OA2=-OA5,OA3=-OA6
故:OA1+OA2+OA3+OA4+OA5+OA6
=(OA1+OA2+OA3)-(OA1+OA2+OA3)
=0