intros.
change (exp_2 (S n0) = 2 * exp_n 2 n0) in |- *.
elim H; auto with arith.
Qed.
Hint Resolve eq_exp_2_exp_n.

Lemma exp_2_n_plus_n : forall n : nat, exp_2 n + exp_2 n = exp_2 (S n).
intro.