用到积化和差公式和倍角公式sinαsinβ=-[cos(α+β)-cos(α-β)]/2cos^2 α=1/2(1+cos2α)sin^2 40°+cos^2 10°-sin40°sin80°=cos^2 50°+cos^2 10°+1/2[cos (40°+80°)-cos(40°-80°)=1/2(1+cos100°)+1/2(1...