Bài 3:Cho tam giác ABC cân tại A. Kẻ AH vuông góc với BC ( H� BC) a) Chứng minh AH là tia phân giác của BAC � b) Kẻ HM vuông góc với AB ( M�AB), kẻ HN vuông góc với AC (N� AC).Chứng minh AM = AN c) Chứng minh MN // BC

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