 reserve n,s for Nat;

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