`cos170^(@)cos150^(@)cos130^@cos110^@=`

Prove that: cos20^(0)cos100^(@)+cos100^(@)cos140^(@)-cos140^(@)-cos140^(@)cos200^(@)=

cos130^(@)+cos110^(@)+cos10^(@)=0

cos130^(@)+cos110^(@)+cos10^(@)=0

Evaluate the following: i) cos120^(@)sin390^(@)+cos330^(@)cos150^(@) ii) sin^(2)(3pi)/(4)+cos^(2)pi/4+sec^(2)pi/3

Evaluate the following: i) cos120^(@)sin390^(@)+cos330^(@)cos150^(@) ii) sin^(2)(3pi)/(4)+cos^(2)pi/4+sec^(2)pi/3

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

sin120^(@)cos150^(@)-cos240^(@)sin330^(@)=

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