intros a b; apply eqA_trans with (y := multA (invA b) a); auto.
apply eqA_trans with (y := invA (multA b a)); auto.
Qed.
Hint Resolve multA_invA_com_r.

Theorem divA_multA_comp_l :
forall (a b c : A) (nZc : ~ eqA c A0),
eqA (divA (multA a b) c nZc) (multA a (divA b c nZc)).