1.cos 20°= ( )
A.cos 30°cos 10°-sin 30°sin 10°
B.cos 30°cos 10°+sin 30°sin 10°
C.sin 30°cos 10°-sin 10°cos 30°
D.cos 30°cos 10°-sin 30°cos 10°
【解析】选B.cos 20°=cos(30°-10°)=cos 30°cos 10°+
sin 30°sin 10°.
2.cos (α-35°)cos (25°+α)+sin (α-35°)sin (25°+α)的值为( )
A.- B. C.- D.
【解析】选B.原式=cos [(α-35°)-(α+25°)]=cos 60°=.
