Consider the following premises. forall x: Q(x). r_3 or s. ~r_3. Can we infer ~s from them? If possible, name the inference rules being used at each step.
 From forall x: Q(x) we can infer Q(a_18) via universal instantiation. Finally, from r_3 or s, ~r_3 we can infer s via disjunctive syllogism, which contradicts ~s. Therefore, the answer is no.