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Cho tam giác ABC có ∠ABC = 60◦, ∠ACB = 45◦, AB = 2a. Vẽ đường cao AH. Gọi D, E là hình chiếu vuông góc của H trên AB, AC. a) Tính BH, BC. b) Chứng minh rằng AD · AB = AE · AC. c) Chứng minh rằng 4 điểm A, D, H, E cùng thuộc một đường tròn. Xác định tâm và tính bán kính của đường tròn đó.
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