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

Lemma Rdiv_plus : forall a b c d : R, b <> 0 -> d <> 0 -> a / b + c / d = (a * d + c * b) / (b * d).