intuition.
unfold ratSubtract.
rattac.
arithNormalize.
inversion H; clear H; subst.
rewrite <- (mult_assoc (n * x * 2)).
rewrite (mult_comm (n * x)).
repeat rewrite mult_assoc.
simpl.
rewrite (plus_0_r).
repeat rewrite mult_plus_distr_r.
remember (n * x * x * 2)%nat as a.
omega.
Qed.

(* Specifications: *)
Lemma ratMult_ratAdd_cd : forall r n (d : posnat),
r + r * (RatIntro n d) == r * (RatIntro (d + n)%nat d).