induction l...
simpl @length.
intros.
simpl Rsum.
apply Rle_trans with (a + INR (length l) * r).
apply Rplus_le_compat_l.
apply IHl.
intros.
apply H.
right...
apply Rle_trans with (r + INR (length l) * r).
apply Rplus_le_compat_r.
firstorder.
simpl @length.
rewrite S_INR.
rewrite Rmult_plus_distr_r.
rewrite Rmult_1_l...
Qed.

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