## Domain Representations

- Lesson

Students use graphs, tables, number lines, verbal descriptions, and symbols to represent the domain of various functions.

Introduce the lesson by having students think about functions in terms of inputs (*x*-values) and outputs (*f*(*x*)-values). Pose the following question:

Is it possible for a function to have an input (x-value) that has no corresponding output (f(x) value)?[Yes, to be a function, each input must have a unique output. However, there are many functions that have a limited set of inputs; for example,

f(x) =x^{½}is only defined forx≥ 0, but each input has a unique output.]

Have students consider the question individually, then with a partner. Instruct students to justify their answer with a graph or an algebraic representation, along with a verbal description. Then, have all students share their answers with a group. (This classroom strategy is known as Think-Pair-Share, and it can be effective in getting students to share their ideas with the entire class. By first testing their ideas with a partner, students are more likely to then share with the entire class.)

After students have considered the possibility that for many functions there exist *x*-values with no corresponding *f*(*x*)-values, distribute the Domain Representations Activity Sheet. Have students read the introduction at the beginning of the sheet.

Domain Representations Activity Sheet

The first representation that students will use is a graph. Students should use a graphing calculator to sketch the graph of each of the eight functions. Have students include all necessary details of each graph, including endpoints, asymptotes, and arrows.

Some students may have difficulty providing appropriate graphs
for functions which contain asymptotes. Explain to students that it is
inappropriate to use a vertical line as part of the function, because
it would no longer be a function. Ask students to investigate the
points at which they think a vertical line occurs; that is, have them
consider the *x*-values and corresponding *f*(*x*)-values at those points

The second representation students will use is a table. Students can use the table function on a graphing calculator to complete the table for each of the eight functions, or they can compute the values by hand using any calculator.

Students may question why an ERROR message appears for some *x*-values
in some of the functions. Students should consider the algebraic
representation for the function. Students should notice that
substitution may result in taking the square root of a negative number
or dividing by zero. When students see an ERROR on the calculator, they
should record the error on their activity sheet as an E.

Once students complete all of the tables, have them answer the following questions:

- Examine the algebraic representations for the functions. Which
functions produce errors in the table? What types of functions have
errors?
[The square root and rational functions produce errors in the tables.]

- Why do square root and rational functions produce errors in the table?
[Taking the square root of a negative number and dividing by zero are both undefined operations, and these operations occur for some

*x*-values.]

The third representation is a number line. Students will plot the *x*-values with corresponding *f*(*x*)-values on a number line.

Explain to students that whenever an *x*-value has a corresponding *f*(*x*)-value on the table, that *x*-value is said to be in the *domain *of the function. The set of all *x*-values with corresponding *f*(*x*)-values is called the domain of the function. Whenever an *x*-value produces an error for an *f*(*x*)-value on the table, that *x*-value is not in the domain of the function.

After students have completed the number line representations,
move students into groups of 2‑4. Have students compare their number
lines with those of their group members. Instruct each group to come to
a consensus with each number line. Assign each group to display a
number line on the board. Review the number lines by asking each group
to justify why they included particular *x*-values.

The fourth and fifth representations—verbal and symbolic. Explain to students that the verbal description can be used to explain the meaning of the symbolic, which is why these representations are presented together.

After students have developed verbal and symbolic
representations, arrange students into new groups of 2‑4. Have students
compare their verbal and symbolic representations with those of their
group members. Instruct each group to come to a consensus with each
representation. Assign each group to put the verbal and symbolic
representations onto the board. Review the verbal and symbolic
representations by asking each group to justify why they included
particular *x*-values.

After reviewing verbal and symbolic representations, students should examine the interconnections between all five representations. They should compare the different representations side-by-side, to see how they are related to one another. For instance, allow students to compare the grapical and number line representations of the following function:

From these two representations, students should recognize that the asymptotes are *x* = ‑2 and at *x* = 3, and holes occur on the number line at ‑2 and 3. Further, errors occur in the table at these same *x*-values:

x | -3 | -2 | -1 | 0 | 1 | 2 | 3 | 4 |

f(x) | 0.17 | E | -0.25 | -0.17 | -0.17 | -0.25 | E | 0.17 |

Finally, the domain of this function could be described verbally as "containing all *x*-values except ‑2 and 3," which symbolically can be represented as either *x* ≠ ‑2, 3, or as (‑∞, ‑2) ∪ (‑2, 3) ∪ (3, ∞).

Use the key questions below and others like them to help students make connections between the five representations for each function.

- Domain Representations Activity Sheet
- Graphing Calculators

**Assessment Options**

- Provide students with the graph of a function. Have students represent the domain of the function with a number line, verbal description, and symbolic representation.
- Given a verbal description for a domain, have students write the symbolic representation and draw a graph of a function with the given domain.
- Consider the function
*f*(*x*) = (*x*+ 2)^{½}. Explain the connections between the graphical, tabular, number line, verbal, and symbolic representations.

**Extensions**

- Ask students to provide algebraic representations for functions with the following domains:
- (-∞, ∞)
- (-∞, 3]
*x*≠ 1

Have students work individually first, then have students move into pairs. In pairs, students will share the functions they invented and explain why they have the given domains.

- Instruct students to respond to the following questions in journal form:
- Will all polynomials have domain (-∞, ∞)? Explain why or why not.
- How does a vertical asymptote at a particular
*x*-value affect the function’s domain? Explain. - Is it possible for a function of the form shown below to have domain (-∞, ∞)?
If possible, give an algebraic representation for

*g*(*x*) and explain why the domain is (-∞, ∞). If not possible, explain why the domain can never be (-∞, ∞) regardless of the algebraic representation for*g*(*x*).

**Questions for Students**

1. Identify the functions with domain (-∞, ∞). Examine the graphical, tabular, number line, verbal, and symbolic representations for each of those functions. When a function has domain (-∞, ∞),

- Will the graph of the function contain an asymptote?
- Will the table of values contain an error?
- What can be determined about the number line?
- How is the function’s domain described verbally?

[The domain is uninterrupted, so there are no asymptotes, no errors in the table of values, and no holes on the number line. The verbal description should mention that the domain contains every possiblex-value.]

2. Identify the functions whose graphs contain asymptotes. Examine the graphical, tabular, number line, verbal, and symbolic representations for each of those functions. When a function’s graph contains an asymptote,

- Will there be an error in the table of values? Explain why.
- What can be determined about the number line? Explain.
- How is the function’s domain described verbally?
- How is the function’s domain described symbolically?

[The asymptotes of the graph occur at the samex-values for which an error occurs in the table, for which holes appear on the number line, and for which the verbal description omits values. Further, the symbolic representation will be divided into two or more pieces, to indicate that somex-values are not in the domain.]

**Teacher Reflection**

- How did the different representations enhance students’ understandings of domain?
- When students worked in groups, were they involved in productive discussion/activity related to the representations?
- What role did the key questions play in students’ development of the connections between different representations?
- To what extent did students verbalize connections between the different representations?

### Learning Objectives

Students will:

- Use graphs, tables, number lines, verbal descriptions, and symbols to represent the domain of various functions.
- Recognize and use connections between a function’s symbolic representation, a function’s graphical representation, and a function’s domain.

### NCTM Standards and Expectations

- Understand relations and functions and select, convert flexibly among, and use various representations for them.

- Analyze functions of one variable by investigating rates of change, intercepts, zeros, asymptotes, and local and global behavior.

### Common Core State Standards – Practice

- CCSS.Math.Practice.MP5

Use appropriate tools strategically.

- CCSS.Math.Practice.MP7

Look for and make use of structure.