standard_description: Understand that a function is a rule that assigns to each input exactly one output. The graph of a function is the set of ordered pairs consisting of an input and the corresponding output. (Function notation is not required in Grade 8.)
problem_activity: Optional activity
Previously, students made sense of the domain of functions in concrete contexts. This optional activity is an opportunity to reason about domain more abstractly. Students evaluate an expression that defines a function at some values of input and notice a value that produces an undefined output. They graph the function to examine its behavior, and then think about how to describe the domain of the function.
Launch
Provide access to graphing technology.
Action and Expression: Provide Access for Physical Action.
Provide access to tools and assistive technologies such as a graphing calculator or graphing software. Some students may benefit from a checklist or list of steps to be able to use the calculator or software.
Supports accessibility for: Organization; Conceptual processing; Attention
Student Facing
Consider the function
\(f(x)=\dfrac {6}{x-2}\)
.
To find out the sets of possible input and output values of the function, Clare created a table and evaluated
\(f\)
at some values of
\(x\)
. Along the way, she ran into some trouble.
Find
\(f(x)\)
for each
\(x\)
-value Clare listed. Describe what Clare’s trouble might be.
|  \(x\)   |   |   |   |   |
|----------|---|---|---|---|
| \(f(x)\) |   |   |   |   |
Use graphing technology to graph function
\(f\)
. What do you notice about the graph?
Use a calculator to compute the value you and Clare had trouble computing. What do you notice about the computation?
How would you describe the domain of function
\(f\)
?
Student Response
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Student Facing
Are you ready for more?
Why do you think the graph of function
\(f\)
looks the way it does? Why are there two parts that split at
\(x=2\)
, with one curving down as it approaches
\(x=2\)
from the left and the other curving up as it approaches
\(x=2\)
from the right?
Evaluate function
\(f\)
at different
\(x\)
-values that approach 2 but are not exactly 2, such as 1.8, 1.9, 1.95, 1.999, 2.2, 2.1, 2.05, 2.001, and so on. What do you notice about the values of
\(f(x)\)
as the
\(x\)
-values get closer and closer to 2?
Student Response
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for free access to Extension Student Response.
Activity Synthesis
Display a graph of the function for all to see. Invite students to share their observations of the behavior of function
\(f\)
based on the completed table and their graph. Solicit their ideas on what the problem might be with this function.
If no students mentioned division by 0 as the issue, bring this up. Ask questions such as:
"What happens when we divide a number by 0?" (The result is undefined.)
"In the expression
\(\dfrac {6}{x-2}\)
, what value or values of
\(x\)
would result in a denominator of 0?" (Only 2 gives a denominator of 0.)
"If 2 does not produce an output, is it a possible input for
\(f\)
?" (No)
Highlight that the domain of
\(f\)
includes all numbers except 2.


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