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: Activity
The purpose of this activity is for students to make connections between different representations of functions and start transitioning from input-output diagrams to other representations of functions. Students match input-output diagrams to descriptions and come up with equations for each of those matches. Students then calculate an output given a specific input and determine the independent and dependent variables.
Launch
Arrange students in groups of 2. Give students 3–5 minutes of quiet work time and time to share their responses with their partner and come to agreement on their answers. Follow with whole-class discussion.
Student Facing
Record your answers to these questions in the table provided.
Match each of these descriptions with a diagram:
the circumference,
\(C\)
, of a circle with
radius
,
\(r\)
the distance in miles,
\(d\)
, that you would travel in
\(t\)
hours if you drive at 60 miles per hour
the output when you triple the input and subtract 4
the volume of a cube,
\(v\)
given its edge length,
\(s\)
Write an equation for each description that expresses the output as a function of the input.
Find the output when the input is 5 for each equation.
Name the
independent
and
dependent variables
of each equation.
[IMG]
|              description               | a | b | c | d |
|----------------------------------------|---|---|---|---|
|                diagram                 |   |   |   |   |
|                equation                |   |   |   |   |
| input = 5                   output = ? |   |   |   |   |
| independent                   variable |   |   |   |   |
|  dependent                   variable  |   |   |   |   |
Student Response
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Student Facing
Are you ready for more?
Choose a 3-digit number as an input.
Apply the following rule to it, one step at a time:
Multiply your number by 7.
Add one to the result.
Multiply the result by 11.
Subtract 5 from the result.
Multiply the result by 13
Subtract 78 from the result to get the output.
Can you describe a simpler way to describe this rule? Why does this work?
Student Response
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Activity Synthesis
The goal of this discussion is for students to describe the connections they see between the different entries for the 4 descriptions. Display the table for all to see and select different groups to share the answers for a column in the table. As groups share their answers, ask:
“How did you know that this diagram matched with this description?” (We remembered the formula for the circumference of a circle, so we knew description A went with diagram D.)
“Where in the equation do you see the rule that is in the diagram?” (The equation is the dependent variable set equal to the rule describing what happens to the independent variable in the diagram.)
“Explain why you chose those quantities for your independent and dependent variables." (We know the independent variable is the input and the dependent variable is the output, so we matched them up with the input and output shown in the diagram.)
Speaking: MLR8 Discussion Supports.
As students describe the connections they noticed in the table across from the different entries for the four descriptors, revoice student ideas to demonstrate mathematical language use. In addition, press for details in students’ explanations by requesting that students challenge an idea, elaborate on an idea, or give an example. This will help students to produce and make sense of the language needed to communicate their own ideas about functions and independent and dependent variables.
Design Principle(s): Support sense-making; Optimize output (for explanation)


aligns: yes