Consider the following premises. p_8 or u. p_8 <-> t_2. u -> r. t_2 -> r. Can we infer ~r from them?
 From p_8 <-> t_2 we can infer p_8 -> t_2. From p_8 -> t_2, t_2 -> r we can infer p_8 -> r. Finally, from p_8 -> r, u -> r, p_8 or u we can infer r, which contradicts ~r. Therefore, the answer is no.