1.3: Intervals and Interval Notation
Suppose you and 2 of your friends were out for lunch and decide to buy tacos. Together you have $15 to spend on lunch, and tacos are $1.25 each. It is clear that the total cost could be graphed as a function of the number of tacos purchased, but how would you specify that the graph should not include values greater than $15 or less than $3.75 (one taco each)?
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Guidance
Real Values and Intervals
A function is defined as a real function if both the domain and the range are sets of real numbers. Many of the functions you have likely encountered before are real functions, and many of these functions have Domain = \begin{align*}\mathbb R\end{align*}
You may already be familiar with the graphs of lines. In particular, you may already be in the habit of placing arrows at the ends. We do this in order to indicate that the line will continue forever in both the positive and negative directions, both in terms of the domain and the range. The line above, however, only shows the function \begin{align*} y=3x\end{align*}
The table below summarizes the kinds of intervals you may need to consider while studying functions and their domains:
Interval notation  Inequality notation  Description 

\begin{align*}\,\! [a,b]\end{align*} 
\begin{align*}\,\! a \leq x \leq b\end{align*} 
The value of x is between a and b, including a and b, where a, b are real numbers. 
\begin{align*}\,\! (a,b) \end{align*} 
\begin{align*}\,\! a < x < b\end{align*} 
The value of x is between a and b, not including a and b. 
\begin{align*}\,\! [a,b)\end{align*} 
\begin{align*}\,\! a \leq x < b\end{align*} 
The value of x is between a and b, including a, but not including b. 
\begin{align*}\,\! (a,b]\end{align*} 
\begin{align*}\,\! a < x \leq b\end{align*} 
The value of x is between a and b, including b, but not including a. 
\begin{align*}(a, \infty)\end{align*} 
\begin{align*}\,\!x > a\end{align*} 
The value of x is strictly greater than a. 
\begin{align*}[a, \infty)\end{align*} 
\begin{align*}\,\!x \geq a \end{align*} 
The value of x is greater than or equal to a 
\begin{align*}(\infty, a)\end{align*} 
\begin{align*}\,\!x <a \end{align*} 
The value of x is strictly less than a 
\begin{align*}(\infty, a]\end{align*} 
\begin{align*}\,\! x \leq a \end{align*} 
The value of x is less than or equal to a. 
Example A
Identify the sets described:
a.) \begin{align*}(3, 9]\end{align*}
b.) \begin{align*}[23, 12]\end{align*}
c.) \begin{align*}(\infty, 0)\end{align*}
Solution
a.) The set of numbers between 3 and 9, ‘‘not including’’ the actual value of 3, but ‘‘including’’ 9.
b.) The set of numbers between 23 and 12, ‘‘including’’ the values 23 and 12.
c.) All numbers less than 0, not including 0 itself.
Example B
Sketch the graph of the function \begin{align*} f(x)=\frac{1}{2}x6 \end{align*}
Solution
The figure below shows a graph of \begin{align*} f(x)=\frac{1}{2}x6 \end{align*}
Example C
Describe the specified intervals, use interval notation:
a.) All positive numbers
b.) The numbers between negative eight and two hundred forty two, including both
c.) All negative numbers, zero, and the positive numbers up to and including nine.
Solution
a.) \begin{align*}(0, +\infty)\end{align*}
 Zero is neither positive nor negative, so the “(“ is used to specify that zero is ‘‘not’’ included. Since there is no maximum positive number, we specify that infinity is the upper value, and use “)” since it cannot be reached.
b.) \begin{align*}[8, 242]\end{align*}
 The “[“ is used on both ends, since both values are included.
c.) \begin{align*}(\infty, 9]\end{align*}
 The “(“ denotes that negative infinity cannot be reached, and “]” on the other end specifies that 9 is included in the set.
Concept question wrapup To specify that the graph of the cost of lunch only includes values between $3.75 and $15.00, specify the interval of the domain as: [3.75, 15]. 

Vocabulary
An interval is a specific part of a function, which may be evaluated separately from the rest of the function.
A closed interval includes the endpoints of the interval.
An open interval does not include the endpoints of the interval.
A real function is a function where both the domain and range are the set of all real numbers.
Guided Practice
Questions
1) Describe the set shown in the image using interval notation
2) Describe the specified intervals, use interval notation:
 a) All negative numbers
 b) The numbers between five and twelve, including five, but not twelve.
 c) Negative numbers down to negative six, zero, and all positive numbers.
3) Describe the domain in the sets in the images using interval notation:
a)  b) 

4) Describe the range in the sets in the images above using interval notation.
Solutions
1) \begin{align*}(\infty, 3) (0, \infty)\end{align*}
 The set is opened with "(", since neg infinity cannot be reached, then closed with ")", since 3 is not included. The set is reopened with "(" since 0 is not included, and finally closed with ")" since pos infinity cannot be reached either.
2) a) \begin{align*}(\infty, 0)\end{align*}
 Zero is neither positive nor negative, so the “)“ is used to specify that zero is ‘‘not’’ included. Since there is no maximum negative number, we specify that infinity is the lower value, and use “(” since it cannot be reached.

b) \begin{align*}[5, 12)\end{align*}
[5,12)  The “[“ is used to open the set, since 5 is included, but ")" is used to close, since 12 is not.

c) \begin{align*}(\infty, 9]\end{align*}
(−∞,9]  The “(“ denotes that negative infinity cannot be reached, and “]” on the other end specifies that 9 is included in the set.
3) a) The domain is the set of x values starting with the included 6 and ending at 4, which is not included: [6, 4)
 b) As above: [6, 7)
4) a) The range is the set of y values from 3 (not included) to 4 (included): (3, 4]
 b) As above: [1, 6)
Practice
Write the following in interval notation.

\begin{align*}3 ≤ x <1\end{align*}
−3≤x<1  0 < x <2
 x > 3
 x ≤ 2
Solve and put your answer in interval notation.
 2x + 3 < 1
 7x + 4 ≤ 2x  6
For each number line, write the given set of numbers in interval notation.
Name the domain and range for each relation using interval notation
Express the following sets using interval notation, then sketch them on a number line
 {x : −1 ≤ x ≤ 3}
 {x : −2 ≤ x < 1}
 A is the set of all numbers bigger than 2 but less than or equal to 5.
 {x: – 3 < x < ∞ }
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Show More 
closed interval
A closed interval includes the minimum and maximum values (endpoints) of the interval.domain
The domain of a function is the set of values for which the function is defined.interval
An interval is a specific and limited part of a function.Interval Notation
Interval notation is the notation , where a function is defined between and . Use ( or ) to indicate that the end value is not included and [ or ] to indicate that the end value is included. Never use [ or ] with infinity or negative infinity.open interval
An open interval does not include the endpoints of the interval.Range
The range of a function is the set of values for which the function is defined.real function
A real function is a function where both the domain and range are the set of all real numbers.Real Number
A real number is a number that can be plotted on a number line. Real numbers include all rational and irrational numbers.Image Attributions
Learn to identify real functions, and recognize closed and open intervals. Learn to interpret and express intervals in interval notation.