theorem Th87:
  spiral_of_fifths(HPS,frequency,Fourth(HPS,frequency)).6
    = (243 qua Real) / 128 * @frequency
  proof
    set MS = HPS;
    reconsider n4 = 5 as Nat;
    set q = Fourth(MS,frequency);
    spiral_of_fifths(MS,frequency,q).n4 is Element of MS;
    then reconsider r32 = (81 qua Real) / 64 * @frequency as Element of MS
    by Th86;
A1: spiral_of_fifths(MS,frequency,q).6
      = spiral_of_fifths(MS,frequency,q).(n4 + 1)
     .= Fifth_reduct(MS,frequency,
            spiral_of_fifths(MS,frequency,q).n4) by Def19
     .= Fifth_reduct(MS,frequency,r32) by Th86;
    consider r,s be positive Real such that
A2: r = r32 & s = (3 qua Real) / 2 * r &
    Fifth(MS,r32) = s by Th54;
A3: ex fr be positive Real st frequency = fr &
    Octave(MS,frequency) = 2 * fr by Def15;
    ((243 qua Real) / 128) * @frequency < 2 * @frequency &
      1 * @frequency < ((243 qua Real) / 128) * @frequency
      by XREAL_1:68;
    then Fifth(MS,r32) is_Between frequency,Octave(MS,frequency) by A2,A3;
    hence thesis by A2,A1,Def18;
  end;
