induction l.
simpl.
rewrite Ravg_nil...
intros.
rewrite Ravg_cons.
unfold Rdiv.
apply Rmult_le_pos...
apply Rplus_le_le_0_compat.
firstorder.
apply Rmult_le_pos...
firstorder.
Qed.

Lemma Ravg_0 (l: list R): (forall x, In x l -> x = 0) -> Ravg l = 0.
Proof with auto.