Answer
A parenthesis is used when the number beside it is not included in the set, while a square bracket is used when the number is included in the set.
Refer to the examples and explanation below.
Work Step by Step
A parenthesis will be used when the number that serves as a boundary (either lower or upper boundary) is not included in the set.
A square bracket will be used when the number that serves as a boundary (either lower or upper boundary) is included in the set.
Examples:
(1) $(3, 9)$ means 3 and 9 are not included in the set
(2) $(3, 9]$ means 3 is not included but 9 is included in the set
(3) $[3, 9]$ means both 3 and 9 are included in the set
