cos20cos40cos60=? sin20sin40sin60=?

题目有误，求值题得数一般是特殊值。原题原题是：
cos20cos40cos80=?
sin20sin40sin80=?

1)由sin2α=2sinacosa得
cos20cos40cos80=2sin20cos20cos80/2sin20
=2sin40cos40cos80/4sin20
=2sin80cos80/8sin20
=sin160/8sin20
=sin20/8sin20=1/8

2)由积化和差公式得
sin20sin40sin80=cos10(cos70cos50)
=cos10·(1/2)[cos(70+50)+cos(70-50)]
=(1/2)cos100·[(-1/2)+cos20]
=(-1/4)cos10+(1/2)cos20cos10
=(-1/4)cos10+(1/2)[cos30+cos10]
=(-1/4)cos10+(√3/4)+(-1/4)cos10=√3/4