Question

write an two collection which represent sets and two which do not represent sets
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Answer 1

Answer:

A set is a collection of objects. The objects are called the elements of the set. If a set has finitely many elements, it is a finite set, otherwise it is an infinite set. ... If two sets A and B have the same elements, we say that they are equal, and write A = B. A subset of a set is a sub-collection of the set.