Vẽ hình, chứng minh chi tiết nhé. Cho đường tròn (O:R) và một điểm A ở ngoài đường tròn. Vẽ hai tiếp tuyến AB, AC với đường tròn (B,C là hai tiếp điểm). Biết OA=2R a. Chứng minh ABOC nội tiếp b. tính số đo góc OAB c. Tính diện tích tam giác ABC theo R d. OA cắt đường tròn tại M. Chứng minh tam giác OBM là tam giác đều e. Vẽ đường kính BD của đường tròn (O:R). Chứng minh OA song song CD

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