has anyone started assignment 2, some question 2 ( Formal proofs) problems, please help if you can?

Its fine for those students in Gauteng who could go to a discussion class, but we haven't had one in Durban yet.

There's not enough examples of each rule in my opinion. Somebody may have stated that comment before but its valid. A study guide with lots of examples would have been more of a help. I got an A for this module on first level and got an A for the first assignment and I really enjoy the subject so I shouldn't have had a problem with this question.

I can now get -Q as a result in my proof. Unfortunately I made use of De Morgan in my proof, and so Fitch says I can't use this step in my proof, although I will probably submit it like this anyway.

My proof is 16 lines long, 2 for premises, 14 for actual proof.

Because Iva and Reanie tried to help me, I too can try help.
Look VERY carefully at page 172, the 'you try it' exercise.

First of all do ^elim from premise 1
Secondly 2 subproofs of both sides of the premise. P is easy to contradict because of premise 1, so see pg 171 last paragraph. Now assume other side of disjuction, and here I used De Morgan to get it into a further disjuction. Again do 2 subproofs, and here follow Pg 172 carefully, as well as the last comment on page 171, trying to get -Q as result again. Now because of v-Elem, can get -Q as final result citing 2 main subproofs and 2nd premise.

Hope this helps. If anyone knows how to prove De Morgan rule as needed above, I would be most grateful. Email me - shaunv@pdc.co.za

Shaun

Sorry - page references are off by 12 pages - was using the PDF version of the book. Subtract 12 from page no.

Can anyone give me the question for question 3? I don't have my textbook with me.

Question 3:
Give an informal proof of the validity of the following arguments.
| b is small unless it’s a cube.
| If c is small, then either d or e is too.
| If d is small, then c is not.
| If b is a cube, then e is not small.
|---
| If c is small, then so is b.

Sorry - at work. Don't have mine either. Did you do Q2.4 - the <--> question? Did you come right witht the above one?
Shaun

Re Q3:
Rewrite them first into FOL, then answer the question.

I need my textbook for 2.4 and for 3. I'll just do them at home.
I got some help with 2.3

Thanks Reanie.

Instead of trying to change your second premise using De Morgan (which Fitch will accept to check the step but not the goal), you need to eliminate the disjunction on the 2nd premise (a BIG help!, you can iterate an assumption inside a Velim subproof) and then instead of using De Morgan, you need to look at the 2nd disjunct and assume the opposite of what you want (conclusion), also look at your &intro rule and if you can prove a contradiction, you can negate the assumption leading to your goal. I hope I could somehow explain it without stating it directly.....

Does anyone know if we can copy print screen pictures from Fitch into our assignment document?

I did.

ShaunGVW Wrote:
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> I did.

Did you do it in the previous assignment as well?

Yes

ShaunGVW Wrote:
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> Yes

Great!

I was worried we would get penalised as the tutorial said we can't use the program files (which is a bit vague). Was getting a bit tired of copying my formal proofs.