Cho tam giác ABC vuông tại A, có AB=AC, H là trung điểm của BC, M và N là hai điểm thuộc BC sao cho BM=CN

  1) CM:\(\Delta ABH=\Delta ACH\)từ đó tính các góc của tam giác ABC

  2) CM: \(\widehat{BAN}=\widehat{CAM}\)

  3) Qua A kẻ đường thẳng xy bất kì sao cho B, C nằm cùng phía đối với xy. Kẻ BD và CE cùng vuông góc với xy (D,E thuộc xy). 

      CM: DE=BD+CE

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2012 . x-2011+ (x-2011)2 = 2013 . 2011-x