
theorem Thm3:
  for a be Real holds sin (3*PI/2 + a) = - cos a
  proof
    let a be Real;
    sin (3*PI/2 + a) = sin (3*PI/2) * cos a + cos (3*PI/2) * sin a
    by SIN_COS:75
    .= - cos a by SIN_COS:77;
    hence thesis;
  end;
