\(C=\dfrac{1}{sin10^0}+\dfrac{\sqrt{3}}{cos10^0}=\dfrac{cos10^0+\sqrt{3}\cdot sin10^0}{cos10^0\cdot sin10^0}\)
\(=\dfrac{\dfrac{1}{2}\cdot cos10^0+\dfrac{\sqrt{3}}{2}\cdot sin10^0}{\dfrac{1}{2}\cdot cos10^0\cdot sin10^0}\)
\(=\dfrac{sin30^0\cdot cos10^0+cos30^0\cdot sin10^0}{\dfrac{1}{4}\cdot sin20}=\dfrac{sin40^0}{\dfrac{1}{4}\cdot sin20^0}\)
\(=\dfrac{2\cdot sin20\cdot cos20}{\dfrac{1}{4}\cdot sin20}=8\cdot cos20\)
=>Chọn C
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