
theorem
  for a be Real holds cos(-2*PI+a)=cos a
  proof
    let a be Real;
    cos(-2*PI + a ) = cos (a - 2*PI)
    .=cos a * 1 + sin a * sin (2*PI) by SIN_COS:77,SIN_COS:83
    .=cos a by SIN_COS:77;
    hence thesis;
  end;
