Cho ̣̣duong tron (O,R) va duong tron (O',R') (voi R > R') cat nhau tai A va B (O va O' nam tren hai mat phang doi nhau bo AB). Duong kinh AC cua (O) cat (O') tai E va duong tron duong kinh AD cua (O') cat (O) tai F (F va E khac diem A).
a/ Chung minh: Ba diem C, B, D thang hang.
b/ Chung minh: Tu giac CFED noi tiep.
c/ Goi K la giao diem cua CF va DE. Chung minh: KA/KB + CA/CE + DA/DF = 2
d/ Ve OM vuong goc CF, ON vuong goc DE, AI vuong goc EF. Chung minh: IB vuong goc MN.