Fixpoint pow2 (n : nat) : nat :=
  match n with
    | O => 1%nat
    | S n => (2 * pow2 n)%nat
  end.

Lemma Rmult_minus_distr_r: forall r1 r2 r3 : R, (r1 - r2) * r3 = r1 * r3 - r2 * r3.