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SectionDomain and Range

SubsectionDomain and Range

Consider the graph of the function \(f (x) =\sqrt{x + 4}\) and observed that \(f (x)\) is undefined for \(x\)-values less than \(-4\text{.}\) For this function, we must choose \(x\)-values in the interval \([-4, \infty)\text{.}\)

square root graph

All the points on the graph have \(x\)-coordinates greater than or equal to \(-4\text{,}\) as shown in Figure114. The set of all permissible values of the input variable is called the domain of the function \(f\text{.}\)

Figure114

We also see that there are no points with negative \(f (x)\)-values on the graph of \(f\text{:}\) All the points have \(f (x)\)-values greater than or equal to zero. The set of all outputs or function values corresponding to the domain is called the range of the function. Thus, the domain of the function \(f (x) =\sqrt{x + 4}\) is the interval \([-4, \infty)\text{,}\) and its range is the interval \([0, \infty)\text{.}\) In general, we make the following definitions.

Domain and Range

The domain of a function is the set of permissible values for the input variable. The range is the set of function values (that is, values of the output variable) that correspond to the domain values.

Using the notions of domain and range, we restate the definition of a function as follows.

Definition of Function

A relationship between two variables is a function if each element of the domain is paired with exactly one element of the range.

SubsectionFinding Domain and Range from a Graph

We can identify the domain and range of a function from its graph. The domain is the set of \(x\)-values of all points on the graph, and the range is the set of \(y\)-values.

Example115
  1. Determine the domain and range of the function \(h\) graphed in Figure116.

  2. For the indicated points, show the domain values and their corresponding range values in the form of ordered pairs.

graph of function
Figure116
Solution
  1. All the points on the graph have \(v\)-coordinates between \(1\) and \(10\text{,}\) inclusive, so the domain of the function \(h\) is the interval \([1, 10]\text{.}\) The \(h(v)\)-coordinates have values between \(-2\) and \(7\text{,}\) inclusive, so the range of the function is the interval \([-2, 7]\text{.}\)
  2. Recall that the points on the graph of a function have coordinates \((v, h(v))\text{.}\) In other words, the coordinates of each point are made up of a domain value and its corresponding range value. Read the coordinates of the indicated points to obtain the ordered pairs \((1, 3)\text{,}\) \((3,-2)\text{,}\) \((6, -1)\text{,}\) \((7, 0)\text{,}\) and \((10, 7)\text{.}\)
graph of function with enclosing rectangle

Figure117 shows the graph of the function \(h\) in Example115 with the domain values marked on the horizontal axis and the range values marked on the vertical axis. Imagine a rectangle whose length and width are determined by those segments, as shown in Figure117. All the points \((v, h(v))\) on the graph of the function lie within this rectangle.

Figure117

The rectangle described above is a convenient window in the plane for viewing the function. Of course, if the domain or range of the function is an infinite interval, we can never include the whole graph within a viewing rectangle and must be satisfied with studying only the important parts of the graph.

  1. Draw the smallest viewing window possible around the graph shown in Exercise118
  2. Find the domain and range of the function.
graph of function
Figure119

Sometimes the domain is given as part of the definition of a function.

Example120

Graph the function \(f (x) = x^2 - 6\) on the domain \(0 \le x \le 4\) and give its range.

Solution

The graph is part of a parabola that opens upward. We obtain several points on the graph by evaluating the function at convenient \(x\)-values in the domain.

\(x\) \(f(x)\)
\(0\) \(-6\) \(\text{since } f(\alert{0})=\alert{0}^2-6=-6\)
\(1\) \(-5\) \(\text{since } f(\alert{1})=\alert{1}^2-6=-5\)
\(2\) \(-2\) \(\text{since } f(\alert{2})=\alert{2}^2-6=-2\)
\(3\) \(3\) \(\text{since } f(\alert{3})=\alert{3}^2-6=3\)
\(4\) \(10\) \(\text{since } f(\alert{4})=\alert{4}^2-6=10\)

The range of the function is the set of all \(f (x)\)-values that appear on the graph. We can see in Figure121 that the lowest point on the graph is \((0, -6)\text{,}\) so the smallest \(f (x)\)-value is \(-6\text{.}\) The highest point on the graph is \((4, 10)\text{,}\) so the largest \(f (x)\)-value is \(10\text{.}\) Thus, the range of the function \(f\) is the interval \([-6, 10]\text{.}\)

parabola with x from 0 to 4
Figure121
Example122

Graph the function \(g (x) = -x+10\) on the domain \(5 \le x \lt 10\) and give its range.

Solution

The graph is part of a line with a negative slope. We obtain several points on the graph by evaluating the function at convenient \(x\)-values in the domain.

\(x\) \(f(x)\)
\(5\) \(5\) \(\text{since } g(\alert{5})=-\alert{5}+10=5\)
\(7\) \(3\) \(\text{since } g(\alert{7})=-\alert{7}+10=3\)
\(10\) \(0\) \(\text{since } g(\alert{10})=-\alert{10}+10=0\)

The range of the function is the set of all \(g (x)\)-values that appear on the graph. We can see in the graph that the highest point is \((5, 5)\text{,}\) so the largest \(g (x)\)-value is \(5\text{.}\) We can also see that the smallest \(g(x)\)-value is as close to (10,0) as we can get coming from above, but not quite reaching (10,0). Since we also have all \(g(x)\)-values between 0 and 10, the range of the function \(g\) is the interval \((0, 5]\text{.}\) Note that the parenthesis is to indicate that we do not quite reach 0 while the bracket on the 5 shows that we achieve the value 5.

line with x from 5 to 10
Figure123

Graph the function \(g(x) = x^3 - 4\) on the domain \([-2, 3]\) and give its range.

Not all functions have domains and ranges that are intervals.

Example125
  1. The table gives the postage for sending printed material by first-class mail in 2016. Graph the postage function \(p = g(w)\text{.}\)

    Weight in ounces \((w)\) Postage \((p)\)
    \(0 \lt w \le 1 \) $0.47
    \(1 \lt w \le 2 \) $0.68
    \(2 \lt w \le 3 \) $0.89
    \(3 \lt w \le 4 \) $1.10
    \(4 \lt w \le 5 \) $1.31
    \(5 \lt w \le 6 \) $1.52
    \(6 \lt w \le 7 \) $1.73

  2. Determine the domain and range of the function.
Solution
  1. From the table, we see that articles of any weight up to \(1\) ounce require $0.47 postage. This means that for all \(w\)-values greater than \(0\) but less than or equal to \(1\text{,}\) the \(p\)-value is \(0.47\text{.}\) Thus, the graph of \(p = g(w)\) between \(w = 0\) and \(w = 1\) looks like a small piece of the horizontal line \(p = 0.47\text{.}\) Similarly, for all \(w\)-values greater than \(1\) but less than or equal to \(2\text{,}\) the \(p\)-value is \(0.68\text{,}\) so the graph on this interval looks like a small piece of the line \(p = 0.68\text{.}\) Continue in this way to obtain the graph shown in Figure126.

    postage step function
    Figure126

    The open circles at the left endpoint of each horizontal segment indicate that that point is not included in the graph; the closed circles are points on the graph. For instance, if \(w = 3\text{,}\) the postage, \(p\text{,}\) is $0.89, not $1.10. Consequently, the point \((3, 0.89)\) is part of the graph of \(g\text{,}\) but the point \((3, 1.10)\) is not.

  2. Postage rates are given for all weights greater than \(0\) ounces up to and including \(7\) ounces, so the domain of the function is the half-open interval \((0, 7]\text{.}\) (The domain is an interval because there is a point on the graph for every \(w\)-value from \(0\) to \(7\text{.}\)) The range of the function is not an interval, however, because the possible values for \(p\) do not include all the real numbers between \(0.3\) and \(1.75\text{.}\) The range is the set of discrete values \(0.47\text{,}\) \(0.68\text{,}\) \(0.89\text{,}\) \(1.10\text{,}\) \(1.31\text{,}\) \(1.52\text{,}\) and \(1.73\text{.}\)

Let \(B(w)\) give the residential water bill in Arid, New Mexico after using \(w\) HCF (hundereds of cubic feet) of water:

\begin{equation*} B(w) = \begin{cases} 30 + 2w\text{, } \amp 0 \le w \le 50\\ 50 + 3w\text{, } \amp w\gt 50 \end{cases}. \end{equation*}

This is an example of a piecewise formula that we will talk more about later. However, for the time being we can try to make sense of the formula to answer the following question.

If the utilities commission imposes a cap on monthly water consumption at \(120\) HCF, find the domain and range of the function \(B(w)\text{.}\)

SubsectionFinding the Domain from a Formula

If the domain of a function is not given as part of its definition, we assume that the domain is as large as possible. We include in the domain all \(x\)-values that make sense when substituted into the function's formula.

For example, the domain of the function \(f (x) =\sqrt{9 - x^2}\) is the interval \([-3, 3]\text{,}\) because \(x\)-values less than \(-3\) or greater than \(3\) result in square roots of negative numbers. You may recognize the graph of \(f\) as the upper half of the circle \(x^2 + y^2 = 9\text{,}\) as shown in Figure128.

graph of upper half circle
Figure128
Example129

Find the domain of the function \(g(x) = \dfrac{1}{x-3}\text{.}\)

Solution

We must omit any \(x\)-values that do not make sense in the function's formula. Because division by zero is undefined, we cannot allow the denominator of \(\dfrac{1}{x-3}\) to be zero. Since \(x - 3 = 0\) when \(x = 3\text{,}\) we exclude \(x = 3\) from the domain of \(g\text{.}\) Thus, the domain of \(g\) is the set of all real numbers except \(3\text{.}\)

  1. Find the domain of the function \(h(x) = \dfrac{1}{(x - 4)^2}\text{.}\)
  2. Graph the function in the window \begin{align} \text{Xmin} \amp = -2 \amp\amp \text{Xmax} = 8\\ \text{Ymin} \amp = -2 \amp\amp \text{Ymax} = 8 \end{align}. Use your graph and the function's formula to find its range.

For the functions we have studied so far, there are only two operations we must avoid when finding the domain: division by zero and taking the square root of a negative number.

Many common functions have as their domain the entire set of real numbers. In particular, a linear function \(f (x) = b + mx\) can be evaluated at any real number value of \(x\text{,}\) so its domain is the set of all real numbers. This set is represented in interval notation as \((-\infty, \infty)\text{.}\)

The range of the linear function \(f (x) = b + mx\) (if \(m \ne 0\)) is also the set of all real numbers, because the graph continues infinitely at both ends. (See Figure131a.) If \(m = 0\text{,}\) then \(f (x) = b\text{,}\) and the graph of \(f\) is a horizontal line. In this case, the range consists of a single number, \(b\text{.}\)

increasing line and horizontal line
Figure131

SubsectionRestricting the Domain

In many applications, we may restrict the domain of a function to suit the situation at hand.

Example132

The function \(h = f (t) = 1454 - 16t^2\) gives the height of an algebra book dropped from the top of the Sears Tower as a function of time. Give a suitable domain for this application, and the corresponding range.

Solution

You can use the window \begin{align} \text{Xmin} \amp = -10 \amp\amp \text{Xmax} = 10\\ \text{Ymin} \amp = -100 \amp\amp \text{Ymax} = 1500 \end{align} to obtain the graph shown in Figure133.

GC graph of falling algebra book function
Figure133

Because \(t\) represents the time in seconds after the book was dropped, only positive \(t\)-values make sense for the problem. The book stops falling when it hits the ground, at \(h = 0\text{.}\) You can verify that this happens at approximately \(t = 9.5\) seconds. Thus, only \(t\)-values between \(0\) and \(9.5\) are realistic for this application, so we restrict the domain of the function \(f\) to the interval \([0, 9.5]\text{.}\)

During that time period, the height, \(h\text{,}\) of the book decreases from \(1454\) feet to \(0\) feet. The range of the function on the domain \([0, 9.5]\) is \([0, 1454]\text{.}\) The graph is shown in Figure134.

graph of falling algebra book function
Figure134

The children in Francine's art class are going to make cardboard boxes. Each child is given a sheet of cardboard that measures 18 inches by 24 inches. To make a box, the child will cut out a square from each corner and turn up the edges, as shown in Figure136.

diagram for making box
Figure136

  1. Write a formula \(V = f (x)\) for the volume of the box in terms of \(x\text{,}\) the side of the cut-out square.
  2. What is the domain of the function? (What are the largest and smallest possible values of \(x\text{?}\))
  3. Graph the function and estimate its range.