 reserve n,s for Nat;

theorem Th75:
  8 * (Triangle n) + 1 = (2 * n + 1) ^2
  proof
    8 * Triangle n = 8 * (n * (n + 1) / 2) by Th19
                  .= 4 * (n^2 + n);
    hence thesis;
  end;
