Bài 3: Tính chiều cao AH của hình thang ABCD( AB//CD), biết AB=7cm, đường trung bình MN = 9cm và diện tích hình thang bằng 45 . Bài 4: Cho tam giác ABC vuông tại A ( AB<AC). Gọi I là trung điểm của cạnh BC. Qua I vẽ IM vuông góc với AB tại M và IN vuông góc với AC tại N. a) Chứng minh tứ giác AMIN là hình chữ nhật b) Gọi D là điểm đối xứng với I qua N. Chứng minh tứ giác ADCI là hình thoi c) Cho AC=20cm, BC=25cm. Tính diện tích tam giác ABC d) Đường thẳng BN cắt DC tại K. Chứng minh: = . Bài 5*: Cho ba số a, b, c thỏa mãn : a+b+c=0. Chứng minh rằng : + − + + = 0.

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