A set is a well-defined collection of distinct objects or elements, typically enclosed in curly braces { }. Sets are foundational in mathematics and are widely used in various topics like probability, algebra, and logic.
Consider the set:
\[ A = \{2, 3, 4, 7, 8, 9, 12\} \]
This means that set A contains the elements 2, 3, 4, 7, 8, 9, and 12.
We can use special symbols to express whether an element belongs to a set or not:
\[ 3 \in A \quad \text{(3 belongs to set A)} \\ 5 \notin A \quad \text{(5 does not belong to set A)} \]