Please help! Have not been helped and want to make sure I’m on the right track

So I’ve watched the sufficient and necessary video and through many frustrating night I think I have it down. And please help, because I’ve commented on a couple and haven’t been given insight, granted I was able to find my answer through going through the other students questions, but help with this one so I know if I have the right approach P: x->z not z->not x P: not D->not z P: ? _________ C: not D->A not A -> D So we have to connect not D->A A is the new variable, so we look for other sufficient conditions that have not D and we see second premise has not D which is not Z then we see not Z is sufficient for not X there fore not X ->A is the missing premise. Sound right?

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Ekene-Ijei September 9, 2020

Hi, Anthony!

I’m going to walk you through how I approach the missing premise drills. Hopefully, it’s helpful.

Step 1: diagram the contrapositives
P1: X->Z
not Z -> not X
P2: not D -> not Z
Z -> D
P3:
C: not D -> A
not A->D

Step 2: identify the sufficient condition of the conclusion and the conclusion’s contrapositive
not D
not A

Step 3: Do either ‘not D’ or ‘not A’ appear in any of the premises?
‘not D’ appears in P2 as ‘not D -> not Z’

Step 4: connect the current premise to the other given premises
not D -> not Z -> not X

Step 5: make what you know sufficient for what you want.
Since we want not D to be sufficient for not A, we simply add ‘A’ to the end of the chain.
Now we have
not D -> not Z -> not X -> A

so by making the missing premise not X -> A, we complete the chain.

Not X -> A = not A -> X is the missing premise.

Writing out the steps initially helped me learn the correct thought process. As time went on, I no longer needed to follow each step explicitly. It comes naturally, so I don’t spend a lot of time writing each step out.