I need help with part (b) of the following question:
To solve this I'm trying to follow a similar worked example here. And the definition of the formal system is here.
So, we know that is a tautology since:
Now, I guess the first step is to show that .
2. .....(instance of tautology )
3. .....(1,2, Modus Ponen)
I'm stuck at this point... How should I continue? Any help is appreciated.