Recall:
Braces look like
$\,\{\;\;\}\,$
and are used for list notation.
Parentheses look like
$\,(\;\;)\,$
and are used in interval notation when an endpoint IS NOT included.
Brackets look like
$\,[\;\;\;]\,$
and are used in interval notation when an endpoint IS included.
EXAMPLES:
Question:
Is
$\,2\,$ in the set
$\,(2,3)\,$?
Solution:
No.
The parenthesis next to the
$\,2\,$ indicates that
$\,2\,$ is not included.
Question:
Is
$\,2\,$ in the set
$\,[2,3)\,$?
Solution:
Yes.
The bracket next to the
$\,2\,$ indicates that
$\,2\,$ is included.
Question:
Is
$\,2.5\,$ in the set
$\,(2,3)\,$?
Solution:
Yes.
The interval
$\,(2,3)\,$ contains all real numbers between $\,2\,$ and $\,3\,$,
but does not include either endpoint.
Question:
Is
$\,2\,$ in the set
$\,\{2,3\}\,$?
Solution:
Yes.
This set has two members:
the number
$\,2\,$, and the number $\,3\,$.
The braces indicate that list notation is being used here.
Question:
Is
$\,100\,$ in the set
$\,\{1,2,3,\ldots\}\,$?
Solution:
Yes.
The “$\,\ldots\,$” indicates that the established pattern continues ad infinitum.
This set contains all positive integers.
Question:
Is
$\,100.5\,$ in the set
$\,\{1,2,3,\ldots\}\,$?
Solution:
No.
The number
$\,100.5\,$ is not an integer.
Question:
Is
$\,100.5\,$ in the set
$\,(2,\infty)\,$?
Solution:
Yes.
This set contains all real numbers strictly greater than $\,2\,$.
Question:
Is
$\,2\,$ in the set
$\,(-\infty,2)\,$?
Solution:
No.
The parenthesis next to the
$\,2\,$ indicates that $\,2\,$ is not included.
Question:
Is
$\,2\,$ in the set
$\,(-\infty,2]\,$?
Solution:
Yes.
The bracket next to the
$\,2\,$ indicates that $\,2\,$ is included.
Question:
Is
$\,1.9999\,$ in the set
$\,(-\infty,2)\,$?
Solution:
Yes.
This interval contains all real numbers less than $\,2\,$.
Master the ideas from this section
by practicing the exercise at the bottom of this page.
When you're done practicing, move on to:
Introduction to Variables