
theorem Thm6:
  for a be Real holds cos (2*PI/3 - a) = - cos (PI/3 + a)
  proof
    let a be Real;
    cos (2*PI/3 - a) = cos (PI - (PI/3+a))
    .= cos PI * cos (PI/3 + a) + sin PI * sin (PI/3 + a) by SIN_COS:83
    .= - cos (PI/3 + a) by SIN_COS:77;
    hence thesis;
  end;
