hehe moi nghi ra cach nay, k biet co dung hay k, mn xem gium :@@a,Trong mp(BND):Q=NI∩BD⇒Q∈(BCD),Q∈(IMN)⇒ Giao tuyen cua (BCD) va(IMN) la duong thang d qua Q va song song voi MN,BC cat CD tai P
⇒ Thiet dien la hinh thang MNPQ
b,
MNPQ la hinh binh hanh ⇒MN=PQ=12BC⇒PQla duong trung binh cua tgBCD
⇒Q la trung diem cua BD
Ap dung dinh li Menelauyt vao tam giac BGD,N,I,Qthang hang:
IBIG.NGND.QDQB=1⇔IBIG.13.1=1⇔IBIG=3hay IBGB=34