=.
答案:B
6.解析:cos 20°+cos 60°+cos 100°+cos 140°
=cos 20°++2cos 120°cos 20°
=cos 20°+-cos 20°=.
答案:
7.解析:sin(α+β)sin(α-β)=(cos 2α-cos 2β)=[(2cos2α-1)-(2cos2β-1)]=cos2β-cos2α=-m.
答案:-m
8.解析:y=+sin x=2
=,
由已知得,所以<≤1.
所以y∈.
答案:
9.解:
=
=
=8cos 10°sin 20°sin 40°=4(sin 30°+sin 10°)sin 40°
=2sin 40°+4sin 40°sin 10°
=2sin 40°-2(cos 50°-cos 30°)=.
10.解:由题设条件知B=60°,A+C=120°,∴,∴.
将上式化简为cos A+cos C=cos Acos C,
则=[cos(A+C)+cos(A-C)].