Consider the following premises. u -> ~w. t_3 -> u_1. u <-> u_1. w. Can we infer t_3 from them? If possible, name the inference rules being used at each step.
 From u -> ~w, w we can infer ~u via modus tollens. From u <-> u_1, ~u we can infer ~u_1 via biconditional elimination. Finally, from t_3 -> u_1, ~u_1 we can infer ~t_3 via modus tollens, which contradicts t_3. Therefore, the answer is no.