What's wrong with this proof for the set C={a1}?

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SUMMARY

The discussion centers on the invalidity of the proof concerning the set C={a1}. The key issue arises from the assertion that C={a1} equals B, which leads to the conclusion that an element a2 from set A is not included in either B or C. This contradiction highlights a flaw in the proof, particularly when it states that set A consists of k+1 numbers, which can be either identical or distinct. The proof's failure is established at this juncture, rendering it invalid.

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Why can't C={a1}?
 
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Which part of
The reason is that C = {a1} = B, so an element of A, namely a2, is not in either B or C.
don't you understand?
 
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The proof clearly falls apart when it is stated let Set ##A## be a set of ##k+1## numbers. These number can either be all the same, or different. Hence the proof is invalid from that point on.
 

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