# Question: prove the following remember to make only one step at...

###### Question details

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!