 reserve n,s for Nat;

theorem Th77:
  n is triangular implies 9 * n + 1 is triangular
  proof
    assume n is triangular; then
    consider k being Nat such that
A1: n = Triangle k;
    1 + 9 * Triangle k = Triangle (3 * k + 1) by Th67;
    hence thesis by A1;
  end;
