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I'm working on an assignment and I'm stuck on this proof. I feel like I'm on the right track but I can't find the way to prove the goal.

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B ^ D

(B^¬A) → ¬C

B → ¬A

(D^E)→ (A v C)

GOAL: ¬E

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  • "on the right track"... What have you tried ? Commented May 12, 2020 at 16:52
  • 2
    The conclusion is a negation; so, a possible strategy is to work by contradiction. Commented May 12, 2020 at 16:52
  • I started by ^elim for B and D, then I did a subproof to start eliminating the conditionals, but how could I prove ~E when ~E is not in the premises? I can't seem to find the logical way to do that.
    – eaglefern
    Commented May 12, 2020 at 17:02
  • 1
    Don't put clarifications in the comments. Please edit your question. @eaglefern. Commented May 13, 2020 at 1:28

1 Answer 1

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I started by ^elim for B and D, then I did a subproof to start eliminating the conditionals, but how could I prove ~E when ~E is not in the premises? I can't seem to find the logical way to do that.

Good start, but you do not need a subproof to eliminate the conditionals. It is an in-context inference.

Okay, now the goal is ¬E when that negation may not be directly derived. That is an indication to try an indirect proof (a proof of negation).

So assume E and seek to derive a contradiction. Let's get you started.

 1.|  B ^ D            premise
 2.|  (B^¬A) → ¬C      premise
 3.|  B → ¬A           premise
 4.|_ (D^E) → (A v C)  premise
 5.|  D                ^elim 1
 6.|  B                ^elim 2
 7.|  ¬A               →elim 3,5
 8.|  B ^ ¬A           ^intro 6,7
 9.|  ¬C               →elim 2,8 
10.|  |_ E             assumption
11.|  |  D ^ E         ^intro 5,10
12.|  |  (A v C)       →elim 4,11
  :|  |  :
  :|  |  :
   |  |  #             ¬elim
   |  ¬E               ¬intro 

Well, obviously Proof by Cases is next.

PS: Some Fitch systems do not include a falsum symbol. Their implementation of negation elimination/introduction may differ somewhat, but the basics are the same. If a contradiction is derivable when E is assumed, then we may infer ¬E.

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  • Hi, I tried Proof by cases, but I keep getting errors
    – eaglefern
    Commented May 13, 2020 at 2:34
  • 1
    It should be straightforward. A contradiction may be derived when assuming the left case (A) because you may reference line 7 (¬A). A contradiction may also be derived when assuming the right case (C) because you can reference line 9 (¬C). Commented May 13, 2020 at 3:15
  • Thank you. I figured it out.
    – eaglefern
    Commented May 13, 2020 at 22:25

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