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vietviet
2 năm trước
Cho hai đường tròn (O) và (O’) cắt nhau ở A và B (O và O’ thuộc hai nửa mặt phẳng bờ AB). Qua A kẻ cát tuyến cắt đường tròn (O) ở C, cắt đường tròn (O’) ở (D). Các tiếp tuyến của hai đường tròn kẻ từ C và D cắt nhau ở I. Chứng minh rằng khi cát tuyến CAD thay đổi thì: a) Góc CBD không đổi. b) Góc CID không đổi.
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