Bài 5 (1 điểm): Cho tam giác đều ABC, hai đường cao BN, CM. a) Chứng minh tứ giác BMNC là hình thang cân b) Tính chu vi của hình thang BMNC, biết chu vi tam giác ABC bắng 24 dm. giúp em bài 5 với ạ !!!!!!!!!!!

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