theorem
  cos(PI/2) = 0 & sin(PI/2) = 1 & cos(PI) = -1 & sin(PI) = 0 &
  cos(PI+PI/2) = 0 & sin(PI+PI/2) = -1 &
  cos(2 * PI) = 1 & sin(2 * PI) =0 by Th75;
