1. ## Proof

[IMG]file:///C:/Users/user/AppData/Local/Temp/moz-screenshot.jpg[/IMG]Can you help how to establish the validity of the argument? (proof steps)

2. I am not sure it is valid. Consider the case when all variables except u are true and u is false.

And in general, what does one mean by "establish the validity of the argument"? There are so many ways to do it, from truth tables to formal proofs, and it completely depends on what instructor's plans for the course are. How were similar exercises solved in the past?

In fact, everybody knows what "and", "or", "not" and "implies" mean. So, even without knowing much technical details, you can try to come up with informal argument that either validates or invalidates the conclusion. Wrapping this informal argument into a form required by the course is often completely straightforward.

3. is it true ?

4. That would be a valid solution provided you know two things: what rule is used at each step and where the mistake is in the problem statement in the original post.