 reserve n,s for Nat;

theorem
  Polygon (s,n) = (s - 2) * Triangle (n -' 1) + n
  proof
    Polygon (s,n) = (s - 2) * (n * (n - 1) / 2) + n
      .= (s - 2) * Triangle (n -' 1) + n by Th21;
    hence thesis;
  end;
