

Báo cáo sai phạm
a)ABCD la hinh binh hanh=>AB//CD va AB=CD
=>AE//FC va AE=FC( vi E,F la trung diem cua AB va CD)
=>AEFC la hinh binh hanh
b)AEFC la hinh binh hanh
=>AF//EC (hay GF//HC) (hay EH//AG)
xet tam giac DHC co GF//HC;G thuoc HD,F la trung diem DC
=> G la trung diem HD=>GH=GD
xet tam giac AGB co EH//AG,H thuoc GB,E la trung diem AB
=>H la trung diem GB=>GH=GB
=>GH=GD=GB
=>DG=1/3DB