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Cho tam giác ABC vuông tại A, AB=15cm, AC=20cm. Kẻ đường cao AH của tam giác ABC a. Chứng minh rằng AB^2= BH.BC, suy ra độ dài các đoạn thẳng BH và CH b. Kẻ HM vuông góc AB và HN vuông góc AC. Chứng minh rằng AM.AB=AN.AC, suy ra tam giác AMN đồng dạng tam giác ACB c. Tính tỉ số diện tích 2 tam giác AMN và ACB, suy ra diện tích tam giác AMN
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