
theorem Thm13:
  tan(PI/6) = sqrt 3 / 3
  proof
    tan(PI/6) = (1/2)/(sqrt(3)/2) by Thm11,Thm12,SIN_COS4:def 1
    .=1/sqrt(3) by XCMPLX_1:55;
    hence thesis by SQUARE_1:33;
  end;
