连AO交BC于D,交EF于G
易证AG=GD AO=2OD
过O作OM和AN垂直BC
OM/AN=OD/AD=1/3
所以SOBC/SABC=OM/AN=1/3 SOBC=2
OEF/OBC=(EF/BC)方=1/4 OEF=1/2