cos20cos40cos80
=(1/2sin20)*(2sin20cos20cos40cos80)
=(1/2sin20)*(sin40cos40cos80)
=1/4*(1/sin20)*(sin80cos80)
=1/8*(1/sin20)*sin160
=1/8*(sin20/sin20)
=1/8
cos20cos40cos80
=(1/2sin20)*(2sin20cos20cos40cos80)
=(1/2sin20)*(sin40cos40cos80)
=1/4*(1/sin20)*(sin80cos80)
=1/8*(1/sin20)*sin160
=1/8*(sin20/sin20)
=1/8