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hienchinhcb
2 năm trước
cho tam giác nhọn ABC ( AB < AC ) nội tiếp ( O;R) đường cao AD , BE và CF của tam giác ABC cắt nhau tại H . Gọi P,Q lần lượt là hình chiếu của D lên các cạnh AB,AC . Chứng minh rằng : a, tứ giác APOQ nội tiếp b , EF//PQ từ đó suy ra tứ giác BPQC nội tiếp
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