 reserve n,s for Nat;

theorem Th76:
  n is triangular implies 8 * n + 1 is square
  proof
    assume n is triangular; then
    consider k being Nat such that
A1: n = Triangle k;
    8 * (Triangle k) + 1 = (2 * k + 1) ^2 by Th75;
    hence 8 * n + 1 is square by A1;
  end;
