
theorem Thm4:
  for a be Real holds cos (3*PI/2 + a) = sin a
  proof
    let a be Real;
    cos (3*PI/2 + a) = cos (3*PI/2) * cos a - sin (3*PI/2) * sin a
    by SIN_COS:75
    .= - (- sin a) by SIN_COS:77;
    hence thesis;
  end;
