theorem Th79:
  REAL_Music is satisfying_fourth_constructible
  proof
  set MS = REAL_Music;
  let frequency be Element of MS;
  consider fr,qr be positive Real such that
A1: fr = frequency and
  qr = (4 qua Real) / 3 * fr and
A2: [fr,qr] in fourth(MS) by Th78;
  fr is Element of MS & qr is Element of MS by Th1;
  hence thesis by A1,A2;
end;
