sin(20°×3)=3sin20°-4sin³20°=√3/2
8sin³20°-6sin20°+√3=0
解此三次方程,得三个实数根,其中0.3420201433.为所求.
cos20°=√(1-sin²20°)=0.9396926208.
tan20°=sin20°/cos20°=0.3639702343.