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diablo2k
2 năm trước
Bài 1: Một cột cờ thẳng đứng có bóng in xuống mặt đất dài 18m (AC=18m), cùng thời điểm đó, một cọc sắt BD cao 2m vuông góc với mặt đất, có bóng in xuống mặt đất là 3m (CB=3m). Tính chiều cao cột cờ. Bài 2: Cho tam giác ABC vuông tại A (AB<AC). Vẽ đường cao AH (H thuộc BC). Lấy điểm D đối xứng với B qua H. a) Chứng minh   ABC HBA ∽ b) Qua C dựng đường thẳng vuông góc với tia AD cắt AD tại E. Chứng minh AH CD CE AD . .  c) Chứng minh   HDE ADC ∽ d) Cho AB=6cm, AC=8cm. Tính diện tích tam giác DEC. e) AH cắt CE tại F. Chứng minh tứ giác ABFD là hình thoi.
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