Câu ` 37 : `

` a, cos 10^@ * cos 30^@ * cos 50^@ * cos 70^@ `

` = 1/2 . [ cos ( 10^@ + 30^@ + 50^@ + 70^@ ) + cos ( 10^@ - 30^@ - 50^@ - 70^@ )] `

` = 1/2 . [ cos 160^@ + cos (-140^@)] `

` = 1/2 . [ cos 160^@ + cos 140^@ ] `

` = 1/2 . ( 0 + 1/2 ) `

` = 3/16 `

` => ` Chọn ` bbC `

` `

Câu ` 38 : `

` cos(π/7) * cos((4π)/7) * cos((5π)/7) `

` = 1/2 . [ cos (π/7) + ((4π)/7) + ((5π)/7) + cos(π/7)-((4π)/7)-((5π)/7)] `

` = 1/2 . [ cos((10π)/7) + cos((-8π)/7)] `

` = 1/2 . ( 0 + 1/4 ) `

` = 1/4 `

` => ` Chọn ` bbC `