 reserve n,s for Nat;

theorem
  for n being non zero Nat holds
    Triangle (2 * n -' 1) = n * (4 * n - 2) / 2
  proof
    let n be non zero Nat;
A1: (2 * n) -' 1 = (2 * n) - 1 by XREAL_1:233,Th1;
    Triangle (2 * n -' 1) = ((2 * n - 1) * (2 * n - 1 + 1)) / 2 by A1,Th19;
    hence thesis;
  end;
