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BÀI 1: Cho tam giác ABC Dựng phía ngàoi tam giác cáchình bình hành ABMN, BCPQ , CAEF. Chứng minh 
BÀI 2: Cho tứ giác ABCD. E,F,G,H là trung điểm của AB,BC,CD,DA . O là trung điểm EG. Chứng minh
a) 
b) 
c) 
BÀI 3: Cho hai hình bình hành ABCD và A'B'C'D'. Chứng minh . BC'D và B'CD' có cùng trọng tâm.
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