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cho nửa đường tròn tâm O đường kính AB .gọi C là 1 điểm trên nửa đường tròn sao cho cung CA <cung CB .Trên nửa mặt phẳng bờ chứa điểm C,kẻ 2 tia Ax và Ay cùng vuông góc với AB tại D và cắt tia Ax tại E .Đường thẳng EC cắt tia By tại F a)cm:BDCF là tứ giác nội tiếp. b)cm:CD2=CE.CF c)Gọi I là giao điểm của AC và DE ;J là giao điểm của BC và DF.cm; IJ//AB c)Khi EF là tiếp tuyến của nửa đường tròn đường kính AB thì D nằm ở vị trí nào trên AB?
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