rewrite plus_mult.
change (2 * exp_2 n = 2 * exp_2 n) in |- *.
reflexivity.
Qed.
Hint Resolve exp_2_n_plus_n.

Lemma exp_2_plus_pn_pn :
forall n : nat, 0 < n -> exp_2 (pred n) + exp_2 (pred n) = exp_2 n.