sin240°=sin(180°+60°）
=-sin60°
=-√3/2
tan(-17π/6)=tan[-2π-π+(π/6)
=tan(π/6)
=√3/3
cos(19π/6)+tan(16π/3)=cos[2π+π+（π/6)]+tan[4π+π+(π/3)
=-(1/2)+√3