Incorrect Answer.
The "proof"
|
|
|
|
1 p  q  r |
premise |
|
|
|
 |
 |
|
 |
 |
 |
|
|
|
 |
|
|
|
2 p |
assumption |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
3 q r |
assumption |
|
|
|
|
|
|
|
4 q |
e1 3 |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
5 q r q |
i 3-4 |
|
|
|
|
|
|
|
6 q |
e 5, 3 |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
7 p q |
i 2-7 |
|
|
|
has a lot of good things going for it. Although one may wonder about
the motivation of assuming q r in
line 3, this is a legitimate action, and a successful one at that,
provided that the rest of the proof is correct. The
only wrong thing about the proof is the invocation of the rule -elimination in line 6. It cites lines
5 and 3 as its arguments and this all type checks beautifully. Alas,
the formula in line 3 is contained in a box that has already
been closed, so this formula can no longer to referred to as
being true!
Back to Question.
Next Question.