cos20°•cos40°•cos60°•cos80°=(  ) A.14 B.18 C.116 D.132

问题描述:

cos20°•cos40°•cos60°•cos80°=(  )
A.

1
4

B.
1
8

C.
1
16

D.
1
32

∵cos20°•cos40°•cos80°=

2sin20°cos20°cos40°cos80°
2sin20°
=
2sin40°cos40°cos80°
22sin20°
=
2sin80°cos80°
23sin20°

∴cos20°•cos40°•cos80°=
1
8
×
sin160°
sin20°
=
1
8
×
sin(180°−20°)
sin20°
=
1
8

又∵cos60°=
1
2

∴cos20°•cos40°•cos60°•cos80°=
1
2
×
1
8
=
1
16

故选C