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2 năm trước
Cho tam giác ABC có 3 góc nhọn nội tiếp đường tròn (O;R) 3 Đường AD,DE,CF, cắt nhau tại H.gọi K là điểm đối xứng H với BC A) chứng minh tứ giác ACKB nội tiếp B)kẽ A A' của (O) chứng minh AA' Vuông.góc với EF C) gọi I là trung điểm của BC; chứng minh H,I,A' thẳng hàng D) gội G là trọng tâm tam giác ABC chứng minh S∆AA'G= 2S AOG
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