 reserve n,s for Nat;

theorem
  for n being non trivial Nat holds
    (1 / 3) * Triangle (3 * n -' 1) = n * (3 * n - 1) / 2
  proof
    let n be non trivial Nat;
A1: (3*n)-'1 = (3*n)-1 by XREAL_1:233,Th1;
    Triangle (3*n-'1) = ((3*n-1) * (3*n-1+1)) / 2 by A1,Th19;
    hence thesis;
  end;
