Cho diem A nam ngoai duong tron (O;hai tiep tuyen AB, AC voi (O) B,C la cac tiep diem) ve duong kinh CD cua (O)
a. CMR: OA vuong goc BC, OA song song BD
b. Goi E la giao cua AD voi (O); H la giao cua OA va BC. CMR: AE.AD=AH.AO
c. CMR: Goc AHE = Goc OED
d. Goi r la ban kinh cua duong tron noi tiep tam giac ABC. Tinh do dai doan thang BD theo R,r
