standard_description: Interpret the equation y = mx + b as defining a linear function, whose graph is a straight line; give examples of functions that are not linear. For example, the function A = s2 giving the area of a square as a function of its side length is not linear because its graph contains the points (1,1), (2,4) and (3,9), which are not on a straight line.
problem_activity: Warm-up
The purpose of this warm-up is for students to use repeated reasoning to write an algebraic expression to represent a rule of a function (MP8). The whole-class discussion should focus on the algebraic expression in the final row, however the numbers in the table give students an opportunity to also practice calculating the square of numbers written in fraction and decimal form.
Launch
Arrange students in groups of 2. Give students 1–2 minutes of quiet work time and then time to share their algebraic expression with their partner. Follow with a whole-class discussion.
Student Facing
Fill in the table of input-output pairs for the given rule. Write an algebraic expression for the rule in the box in the diagram.
[IMG]
{"table": [{"_attributes": {"border": "1"}, "thead": [{"tr": [{"th": [{"_attributes": {"scope": "col"}, "_value": "input"}, {"_attributes": {"scope": "col"}, "_value": "output"}]}]}], "tbody": [{"tr": [{"td": [{"_value": "8"}, {}]}, {"td": [{"_value": "2.2"}, {}]}, {"td": [{"span": [{"_attributes": {"class": ["math", "math-repaired"], "data-png-file-id": "40402"}, "_value": "\\(12\\frac14\\)"}]}, {}]}, {"td": [{"span": [{"_attributes": {"class": ["math", "math-repaired"], "data-png-file-id": "833"}, "_value": "\\(s\\)"}]}, {}]}]}]}]}
Student Response
Teachers with a valid work email address can
click here to register or sign in
for free access to Student Response.
Activity Synthesis
Select students to share how they found each of the outputs. After each response, ask the class if they agree or disagree. Record and display responses for all to see. If both responses are not mentioned by students for the last row, tell students that we can either put
\(s^2\)
or
\(A\)
there. Tell students we can write the equation
\(A = s^2\)
to represent the rule of this function.
End the discussion by telling students that while we’ve used the terms input and output so far to talk about specific values, when a letter is used to represent any possible input we call it the
independent variable
and the letter used to represent all the possible outputs is the
dependent variable.
Students may recall these terms from earlier grades. In this case,
\(s\)
is the independent variable and
\(A\)
the dependent variable, and we say “
\(A\)
depends on
\(s\)
.”


aligns: no