1.tg(20пи/3)=
2.tg(28пи/3)=
3.tg(-31пи/6)=
4.ctg(16пи/3)=

1.  =tg(21π/3  - π/3)=tg(7π - π/3)= -tg(π/3)= -√3
2.  = -tg(31π/6) = - tg(30π/6  + π/6)= -tg(5π + π/6) =
     = -tg(π/6)= - 1/√3 = -√3/3
3.  =ctg(15π/3  + π/3)=ctg(5π + π/3) = ctg(π/3)= 1/√3 = √3/3