Bài 65 trang 219 SGK Đại số 10 Nâng cao

Chọn đáp án đúng

({{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} )

Đáp án

Ta có:

({{cos {{80}^0} - cos {{20}^0}} over {sin {{40}^0}cos {{10}^0} + sin {{10}^0}cos {{40}^0}}} = {{ - 2sin {{50}^0}sin {{30}^0}} over {sin ({{40}^0} + {{10}^0})}} )

(=  - 2sin {30^0} =  - 1) 

Vậy chọn C

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