2(cos10°)^2-tan5°（1+cos10°）-2sin40°sin80° =cos20°+1-sin5°/cos5°*(1+2(cos5°)^2-1)-(cos(40°-80°)-cos(40°+80°))=cos20°+1-2sin5°*cos5°-cos40°+cos120°=cos20°-sin10°-cos40°+1/2=cos20...