Bài 65 trang 219 SGK Đại số 10 Nâng cao

Đề bài

\({{\cos {{80}^0} - \cos {{20}^0}} \over {\sin {{40}^0}\cos {{10}^0} + \sin {{10}^0}\cos {{40}^0}}}\) bằng:

\(\eqalign{
& (A)\,\,1\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,(B)\,{{\sqrt 3 } \over 2} \cr 
& (C)\,\, - 1\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,(D)\, - {{\sqrt 3 } \over 2} \cr} \)

Lời giải chi tiết

Ta có:

\({{\cos {{80}^0} - \cos {{20}^0}} \over {\sin {{40}^0}\cos {{10}^0} + \sin {{10}^0}\cos {{40}^0}}} \)

\(\begin{array}{l}
= \frac{{ - 2\sin \frac{{{{80}^0} + {{20}^0}}}{2}\sin \frac{{{{80}^0} - {{20}^0}}}{2}}}{{\sin \left( {{{40}^0} + {{10}^0}} \right)}}\\
= \frac{{ - 2\sin {{50}^0}\sin {{30}^0}}}{{\sin {{50}^0}}}\\
= - 2\sin {30^0}\\
= - 2.\frac{1}{2}\\
= - 1
\end{array}\)

Vậy chọn C

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