Cho hình thang ABCD có AD = BD. Qua B kẻ đường thẳng song song với AC, cắt đường DC tại E. Chứng minh rằng : a, Tam giác BDE là tam giác cân b, Tam giác ACD = tam giác BDC c, Hình thang ABCD là hình thang cân

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