First time posting here. I'm going through some examples and stumbled upon $¬A \land (A \lor B) \vdash B$.
I'm hoping someone can help me with understanding it. This is what I've done so far, mostly I don't know how to continue from line 9 onwards taking into consideration the first part is correct.
TIA
$¬A \land (A \lor B) \vdash B$
1. ¬A ^ (A v B) hyp
2. |¬B ass
3. |¬A 1, ^E1
4. ¬B => ¬A 2-3, =>I
5. |¬B ass
6. | |¬A sub ass
7. | |¬B copy 5.
8. |¬A => ¬B 6-7, =>I
9. | |¬A sub ass
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a. |¬A => B
b. |¬¬A 8,a, ¬I
c. |A b, ¬E
d. ¬B => A 5-c, =>I
e. B 4,d ¬I
Update. Tried it like this. Does it make sense? The only issue I have is with line 8 ...
1. ¬A ^ (A v B) hyp
2. |¬B ass
3. |A v B 1, ^E2
4. ¬B => A v B 2-3, =>I
5. |¬B ass
6. |¬A 1, ^E1
7. |¬A v ¬B 6, vI1
8. |¬(A v B) (Taking not common)
9. ¬B => ¬(A v B) 5-8, =>I
10. ¬¬B 4,9 ¬I
11. B 10, ¬E
First step: we have to "unpack" the premise : $\lnot A \land (A \lor B)$ using $\land$-elim.
Thus, the derivation must start with :
1) $\lnot A \land (A \lor B)$ --- premise
2) $\lnot A$ --- from 1) by $\land$-elim
3) $(A \lor B)$ --- from 1) by $\land$-elim.
Then we have to apply $\lor$-elim to 3) :
4) $A$ --- assumed from 3) for $\lor$-elim
5) $B$ --- form the contradiction of 2) with 4) by Ex falso
6) $B$ --- assumed from 3) for $\lor$-elim
Having derived $B$ under both assumptions 4) and 6), we have