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Cho tam giác ABC nhọn
[TEX]\frac{1}{cosA}+\frac{1}{cosB}+\frac{1}{cosC}=\frac{1}{sin{\frac{A}{2}}}+\frac{1}{sin{\frac{B}{2}}}+{\frac{1}{sin{\frac{C}{2}}}[/TEX]
cm tam giác ABC đều
[TEX]\frac{1}{cosA}+\frac{1}{cosB}+\frac{1}{cosC}=\frac{1}{sin{\frac{A}{2}}}+\frac{1}{sin{\frac{B}{2}}}+{\frac{1}{sin{\frac{C}{2}}}[/TEX]
cm tam giác ABC đều
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