角a （-3/5,4/5）
x=-3/5 y=4/5 r=根号(x^2+y^2)=1
sina=y/r=4/5 cosa=x/r=-3/5 tana=y/x=-4/3
sin(11π/6)=sin(2π-π/6)=-sinπ/6=-1/2
cos(11π/6)=cos(2π-π/6)=cosπ/6=√3/2
tan(11π/6)=sin(11π/6)/cos(11π/6)=-√3/3