原式=cos80*1/2*cos40*cos20
=2sin20cos20cos40cos80/4sin20
=sin40cos40cos80/4sin20
=sin80cos80/8sin20
=sin160/16sin20
=sin20/16sin20
=1/16
原式=cos80*1/2*cos40*cos20
=2sin20cos20cos40cos80/4sin20
=sin40cos40cos80/4sin20
=sin80cos80/8sin20
=sin160/16sin20
=sin20/16sin20
=1/16