
theorem Th2:
  for a being Real holds cos (3*PI - a) = -cos a
  proof
    let a be Real;
    cos(3*PI - a) = cos (2*PI + (PI -a))
                 .= cos (PI - a) by SIN_COS:79
                 .= - cos a by EUCLID10:2;
    hence thesis;
  end;
