
theorem
  sin(-PI/6) = -1/2 & cos(-PI/6) = sqrt 3 / 2  & tan(-PI/6) = -sqrt 3 / 3 &
  sin(-PI/3) = -sqrt 3 / 2 & cos(-PI/3) = 1/2 & tan(-PI/3) = -sqrt 3
  by SIN_COS:31,SIN_COS4:1,Thm8,Thm9,Thm11,Thm12,Thm10,Thm13;
