cho duong tron (O;R) , OA=3R. Tu Ave hai tiep tuyen AB,AC cua (O) (B,C la hai tiep tuyen) ve day BD song song AC, BD cat CO tai E , OA cat BC tai H a) cm tu giac ABOC noi tiep va BC la phan giac ABD B) cm OA vuong goc BC va CO vuong goc BD roi suy ra tu giac OHBE noi tiep C) goi M la giao diem cua AD voi (O) (M khac D), tia BM cat AC tai N cm NC^2=NM.NB va N laf trung diem cua AC d) goi I,J,K lan luot la ba diem tren ba doan thang BC,CA,AB sao cho goc JIK= goc ABC cm BK.CJ<= BC^2/4
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