 reserve n,s for Nat;

theorem Th83:
  1 / (Triangle n) = 2 / (n * (n + 1))
  proof
    1 / (Triangle n) = 1 / ((n * (n + 1)) / 2) by Th19
      .= 2 / (n * (n + 1)) by XCMPLX_1:57;
    hence thesis;
  end;
