Hint Resolve Qlt_plus_pos_pos.

Lemma Qopp_linear :
forall (a b : Z) (q : Qpositive),
Qplus (Qmult (Z_to_Q (- a)) (Qpos q)) (Z_to_Q (- b)) =
Qopp (Qplus (Qmult (Z_to_Q a) (Qpos q)) (Z_to_Q b)).