Section Ring1.
Context {K : Ring}.
Definition mult : K -> K -> K := Ring.mult _ (Ring.class K).
Definition one : K := Ring.one _ (Ring.class K).

Lemma sum_n_mult_r :
forall (a : K) (u : nat -> K) (n : nat),
sum_n (fun k => mult (u k) a) n = mult (sum_n u n) a.