I thought that the contra positive of A->B was not B->not A? The explanation shows the contra positive as B-some-A. Does the third premise of "at least one A exists" cause this change. I'd really like to understand this. Which lecture covers this?
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Yes, the key here is the third premise which tells us that "at least one A exists." Without this premise, you would not be able to make this deduction.
This concept is discussed in our Quantifiers lesson.
Hope this helps! Please let us know if you have any other questions.
