cho tam giác ABC cân tại A. Kẻ AH vuôn góc với BC tại H. a, Cho biết AB = 10cm, BC= 8cm. Tính độ dài đoạn thẳng BH b, Chứng minh tam giác HAB và tam giác HCA c, Gọi D là điểm nằm trên đoạn thẳng AH. Trên tia đối của tai DB lấy điểm E sao cho DE = DB. Chứng minh rằng: AD+DE>AC d, Gọi K là điểm trên đoạn thẳng CD sao cho CK = 2/3 CD. Chứng minh rằng ba điểm H,K,E thẳng hàng.

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