It is important only to learn a part of it. Nobody can ever learn the set because it is infinite.
The se is used for counting and that is one of the very basic applications of mathematics.
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Every mathematician uses/used integers. It is one of the most important number sets.
It is a universal set
Because otherwise you would only know how to subtract and add with the same integers. Not a particularly useful skill unless just those integers turned up!
Not necessarily. The odd integers and the even integers are two infinitely large sets. But their intersection is the null (empty) set.
the answer is -1