`A=cos20^@ cos40^@ cos60^@ cos80^@ , B=cos60cos42^@ cos66^@ cos78^@` and `C=cos36^@ cos72^@cos 108^@ cos 144^@` then

cos20^@cos40^@cos80^@=

A=cos20^(@)cos40^(@)cos60^(@)cos80^(@),B=cos60cos42^(@)cos66^(@)cos78^(@) and C=cos36^(@)cos72^(@)cos108^(@)cos144^(@) then

cos 36^@ - cos72^@ =

A=cos 20^(@)cos 40^(@) cos60^(@)cos 80^(@) , B=cos6^(@)cos42^(@)cos66^(@)cos78^(@) and C=cos 36^(@)cos72^(@)cos 108^(@)cos 144^(@) then

cos 20^(@) cos 40^(@) cos 80^(@)=

cos6^(@) cos42^(@) cos66^(@) cos78^(@) =

cos20 ^ (@) cos40 ^ (@) cos80 ^ (@) =

cos20^(@).cos40^(@).cos60^(@).cos80^(@) is :

cos20^(@).cos40^(@).cos60^(@).cos80^(@)=1/16