induction l; intros.
simpl.
field.
simpl Rsum.
simpl @length.
rewrite IHl.
rewrite (H a).
rewrite S_INR.
field.
left...
intros.
apply H.
right...
Qed.

Lemma Rsum_le l r: (forall x, In x l -> x <= r) -> Rsum l <= INR (length l) * r.
Proof with auto with real.