1.cos 165°的值是( D )
(A) (B)
(C) (D)
解析:cos 165°=cos(180°-15°)
=-cos 15°
=-cos(45°-30°)
=-cos 45°cos 30°-sin 45°sin 30°
=-×-×
=,故选D.
2.cos 75°cos 15°-sin 255°sin 15°的值是( B )
(A)0 (B) (C) (D)-
解析:原式=cos 75°cos 15°+sin 75°sin 15°
=cos(75°-15°)
=cos 60°=.故选B.
3.已知α为锐角,β为第三象限角,且cos α=,sin
