Cho tam giác ABC có AB <AC , AH là đường cao. Gọi M, N, K lần lượt là trung điểm của AB, AC, BC. a) Chứng minh MNKH là hình thang cân. b) Trên tia AH và AK lần lượt lấy điểm E và D sao cho H là trung điểm của AE và K là trung điểm của AD. Chứng minh tứ giác BCDE là hình thang cân.

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