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vuminhngoc181
2 năm trước
Cho 3 điểm A,B,C phân biệt thẳng hàng theo thứ tự đó. Vẽ (o) bất kì đi qua 2 điểm B,C (O KHÔNG THUỘC BC). Gọi E, F là các tiếp điểm của các tiếp tuyến kẻ từ A tới (o). gọi M là trung điểm của BC. b) Gọi H là giao điểm của hai đường thẳng AO và EF. Gọi K là giao điểm của hai đường thẳng Bc và EF. CM : $\frac{2}{AK}$= $\frac{1}{AB}$ +$\frac{1}{AC}$
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