
theorem Thm2:
  for a be Real holds sin(-2*PI+a) = sin a
  proof
    let a be Real;
    sin(-2*PI + a ) = sin (a - 2*PI)
    .=sin a * 1 - cos a * sin (2*PI) by SIN_COS:77,SIN_COS:82
    .=sin a by SIN_COS:77;
    hence thesis;
  end;
