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Blast into Math! ets of nummers: mathematical plaagrounds

then we write

S \{a 4 }

to mean the set S with the element a 4 removed. So,

S \{a 4 } = {a 1 ,a 2 ,a 3 ,a 5 }.

Definition 3.1.7. Let A and B be sets. Then, the set

A \ B

is the set which contains the elements of A that are not in B , and is pronounced “A minus B .” Symbolically,

A \ B = {x ∈ A such that x/∈ B}.

3.2 Numbers

The most fundamental set of numbers is the set of natural numbers, which are also known as whole
numbers or counting numbers, since they are the numbers we use to count things.

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