cho đường tròn (O) đường kính BC=2R (R không đổi) Lấy điểm A thuộc đường tròn (O) ( A khác B,C ) kẻ AH vuông góc với BC tại H. Vẽ đường tròn (I) đường kính AH cắt AB,AC lần lượt tại M , N. a, Chứng minh: MN=AH b,Chứng minh: BMNC nội tiếp và 1/HN^2 + 1/HM^2 = BC^2/AH^2 c,Kẻ đường kính AD của đường tròn ngoại tiếp tứ giác BMNC. Tìm vị trí của A trên đường tròn (O) sao cho đường tròn ngoại tiếp tứ giác BMNC có bán kính lớn nhất.

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