cos20°－cos40°－cos80°
= cos(30°-10°) - cos(30°+10°) - sin10°
=cos30°*cos10° + sin30°*sin10° - cos30°cos10°+ sin30°sin10° - sin10°
= 2sin30°sin10° - sin10°
= 2* (1/2) * sin10° - sin10°
= sin10° - sin10°
= 0