Question: prove the following remember to make only one step at...
Prove the following. Remember to make only one step at a time
and not skip steps. Also, remember to
cite the rule that you apply for each step, and which lines you applied it to
L1: P ⇒ Q
L2: ¬P ⇒ R
L3: Q ⇒ (S ∧ ¬S)
Goal: (R ∨ U)
please explain every steps, especially the last part... thank you!