11.20 Number 7

Hey everyone,

I am having trouble translating sentence 7 on 11.20. This is the one where we are asked to translate “Only dodecahedra are larger than everything else”. I am unsure of how exactly to translate “only”. At first I had ∀x ∀y (Larger(x, y) → Dodec(x)), but it keeps saying it can’t determine if this is correct. If anyone has any ideas that would be great! Thanks!

11.17 Number 9

Hi all,

I’m having trouble translating sentence 9 in 11.17, which says that every dodecahedron with nothing to its right has something to its left. Right now, I am saying there exists an x, where if x is a dodecahedron AND there doesn’t exist a y that is right of x, then there exists some z such that z is left of x. My sentence isn’t “equivalent to any of the expected answers.” Does anyone see where I went wrong?

Quantifiers tip!

I had a little bit of a goof up when using quantifiers, so I thought I’d tell you all to make sure nobody else stumbles into this. It’s pretty simple, but it tripped me up at first.

When using two variables at the same time, do not put both variables under the same quantifier, like this: Ɐxy . Instead, make sure to use two quantifiers and have one variable per quantifier, like this: Ɐx Ɐy . Hope this helps!

6.22 Issue

Hi everyone,

I’m having an issue with problem 6.22. I can prove ¬(b=c) in step 11 by using ¬Intro and citing 3-10, but for some reason Fitch won’t give me the final check for ¬(b=c) in my goal box. Meaning, every step from 1-11 gets the green check but for some reason the proof as a whole does not. Anyone running up against this issue or have an idea about what I’m doing wrong?

Error help

Hey guys, I keep getting this error in grade grinder even though Boole says my sentences are correct, and I was wondering if anyone recognizes it:

“Your target sentences are wrong.  Please reread the exercise
         carefully.”

Let me know if you’ve seen this!

Help Understanding 3.23

I struggled with sentence 3 of 3.23 until I went back and followed the example FOL translation for either/or, so I know the correct answer, but I don’t understand why that is the correct correct answer.

The sentence is: Folly belonged to either Max or Claire at 2:05 pm.

The translation was: Owned(max, folly, 2:05) ∨ Owned(claire, folly, 2:05)

Earlier the book made a point that “or” in English is sometimes an exclusive or, whereas in logic, disjunction is an inclusive or, meaning that one or both sentences are correct. In English, “Folly belonged to either Max or Claire at 2:05 pm” says to me that Folly belonged to one or the other and not both, but this remains an option in the correct FOL translation. In English-to-FOL translations, are we to assume that unless stated explicitly that both statements cannot be true, every “Either/or” is an inclusive or? Thanks in advance.

Exercise 3.18 Question

I am struggling to find a sentence that is equivalent to (a = d) without using the identity symbol. I know I need to find a sentence that can not be true of two different blocks, but I am not sure I understand how else to indicate that one block has two names. Is there another way I should be thinking about this sentence?

Homework Question

Hi Everyone,

I have a question about the homework that was due this morning at 6. For exercise 2.20 Grade Grinder kept giving me the feedback that my proof was correct but didn’t match the steps of the informal proof on page 52. When it says the proof should contain three steps matching the informal proof, I thought that would mean three steps after the Fitch line, so in addition to the three premises. Grade Grinder seems to be telling me that I shouldn’t have three non-premise steps. I’m wondering if the issue is actually about the rules I cited for each steps. Does anybody have an idea about which problem I was running up against? Thanks, Sara