Interval Notation
Interval notation is a way of writing subsets of the real number line.
A closed interval is one that includes its endpoints: for example, the set {x | –3 
x 1}.
To write this interval in interval notation, we use closed brackets [ ]:
[ –3, 1]
An open interval is one that does not include its endpoints, for example, {x | –3 
x 1}.
To write this interval in interval notation, use parentheses:
( –3, 1)
You can also have intervals which are half-open and half-closed:
[ –2, 4)
You can also use interval notation together with the set union operator to write subsets of the number line made up of more than one interval:
