Nội dung bài giảng
\(\cos {\pi \over {12}}\cos {{7\pi } \over {12}}\) bằng:
\((A)\,{{\sqrt 3 } \over 2}\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,(B)\,{{\sqrt 3 } \over 4}\)
\((C)\,{1 \over 2}\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\;\;\;\;\;\;\;(D)\, - {1 \over 4}\)
Đáp án
Ta có:
\(\cos {\pi \over {12}}\cos {{7\pi } \over {12}} = {1 \over 2}(cos{{2\pi } \over 3} + \cos {\pi \over 2}) = - {1 \over 4}\)
Chọn D