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5.1 and 5.2 of May/June 2010

Posted by sheepapple 
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5.1 and 5.2 of May/June 2010
November 07, 2010 12:18PM
Anyone get the answer 100% right?
Re: 5.1 and 5.2 of May/June 2010
November 11, 2010 11:47PM
5.1 was easy,
5.2 was a bit more challenging but doable (I got stuck for a while at the end since I had not played that much with contradictions and quantifiers, but if you had spent some time doing the questions in the text book, this should have been fairly easy – most students would have gotten more than half the marks though)
Re: 5.1 and 5.2 of May/June 2010
November 14, 2010 02:36PM
I can't for the life of me figure out 5.2.

Since the premise is NOT Ex (Angry(x) ^ Pretty(x)), what should our first subproof be? Is it NOT(Angry(e) ^ Pretty(e))? Is it NOT Angry(e) OR NOT Pretty(e).

The negation in the premise is really throwing me off. I can't find a simlar problem in teh textbook or study guide...don't know what to do with it!
Re: 5.1 and 5.2 of May/June 2010
November 14, 2010 04:47PM
First sub proof starts with b
then you can infer
Funny(b) v Clever(b) v Pretty(b)
as well as
(Funny(b) v Clever(b)) -> ~Angry(b)

both are sited by Universal Elimination

Now, clearly we have to prove by case (3 in fact)

Sub proof one ( Assume Funny(b)) should not be a problem (you wish to get ~Angry(b) as end result)
Sub proof two (Assume Clever(b)) should also be easy (you wish to get ~Angry(b) as end result)

Sub proof three is the tricky one..
Assume Pretty(b)
Now make a new sub proof under this one and assume Angry(b).
Now, you can state Angry(b) & Pretty(b) (Conditional Intro from Pretty(b) and Angry(b))
Since we now have Angry(b) & Pretty(b), we can state Ex(Angry(x) & Pretty(x)), but guess what, this contradicts Premise 1, so indicate contradiction and state your two lines, end the sub proof and now state ~Angry(b) with rule ~Intro and state the lines of the sub proof you just finished. Now, you should see that you have three cases and all of them ends with ~Angry(b), so end this last case and write ~Angry(b). State as rule Disjunction elimination and state the lines for your disjunction sentence and your three cases. End the sub proof and state Ax ~Angry(x) and state as rule, Universal Intro specifying the lines for your b sub proof
And that's that..
Re: 5.1 and 5.2 of May/June 2010
November 14, 2010 05:13PM
Ooooh I see. So you don't actually assume anything next to the little block (b in this case) in your first subproof? Man, I couldn't find a single example like this anywhere! (At least not one we have the answer to).

That's pretty tricky imo, considering ALL the examples in the assigmnet have you assuming something next to the little block thing in your subproof first.

Thanks a lot man!
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