cho doan thang AB ,O la trung diem,tren cung 1 nua mat phang bo AB,ke 2 tiep tuyen Ax,By vuong voi AB.tren à va By lay 2 diem C,D sao cho goc COD bang 90 do.OD keo dai cat AC tai I
a/OD = OI
b/CD la tiep tuyen cua duong tron tam O duong kinh AB
c/CD = AC+BD