This Warm-up prompts students to compare four equations. It gives students a reason to use language precisely (MP6). It gives the teacher an opportunity to hear how students use terminology and talk about characteristics of the items in comparison to one another. In particular, encourage students to be clear about any connections they make between the equation and what the corresponding graph must look like.
Monitor for any students who use the distributive property to rewrite the first equation in standard form.
Launch
Arrange students in groups of 2–4. Display the equations for all to see. Give students 1 minute of quiet think time, and ask them to indicate when they have noticed three equations that go together and can explain why. Next, tell students to share their response with their group and then to work together to find as many sets of three as they can.
Activity
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Student Task Statement
Which three go together? Why do they go together?
A
B
C
D
Activity Synthesis
Invite each group to share one reason why a particular set of three go together. Record and display the responses for all to see. After each response, ask the class if they agree or disagree. Since there is no single correct answer to the question of which three go together, attend to students’ explanations and ensure that the reasons given are correct.
During the discussion, ask students to explain the meaning of any terminology they use, such as "constant," "term," or "coefficient," and to clarify their reasoning as needed. Consider asking:
“How do you know ?”
“What do you mean by ?”
“Can you say that in another way?”
If not brought up during the discussion, select previously identified students to share how they rewrote the first equation, including any organizing techniques they used, such as a diagram.
This activity prompts students to notice the structure that relates expressions in factored form to their equivalent counterparts in standard form. When translating between these structures, it can be helpful to have a way to visually organize the work. Use this activity if students would benefit from additional practice using diagrams to organize their multiplication.
When working with negative numbers, such as linear factors of the form , it is helpful to think of subtracting as adding , and labeling the diagram accordingly. For example, for the last expression, , one side of the diagram should be labeled with and -1, and the other labeled with and -7.
This activity asks students to multiply two factors together to rewrite polynomials in standard form, scaffolding the level of difficulty to prepare students for the upcoming activity.
Display both equation forms and the graph of for all to see.
Here are some questions for discussion.
“Where can you see the 240 in the factored form of the equation? In the graph?” (240 is the constant term, which is the result of multiplying the 4, -6, and -10 values in the factored form. The graph intercepts the vertical axis at 240.)
“What advantages and disadvantages does each form have when considering a cubic polynomial?” (In factored form, the factors help you identify the zeros, but then you need to do a bit of work to figure out the vertical intercept and the degree. In standard form, the vertical intercept and degree can be easier to see, but the zeros are not as obvious.)
“Let’s say is a polynomial with the same zeros and degree as , but with a vertical intercept at instead of . What could the equation for be?” ()
Student Lesson Summary
We can express polynomials in different, yet equivalent, algebraic forms that each give us different information about features of the polynomial and its graph. Earlier, we saw how expressing a polynomial function in factored form is helpful for identifying zeros. The expanded version of factored form, or standard form of a polynomial, is helpful for identifying the constant term and the degree.
For example, here are the expressions for a polynomial written in factored form and standard form:
The constant term, seen as -4.5 in the example, tells us the value of the function when . In a graph of the function, this point is known as the vertical intercept.
The degree, seen as 5 in the example, tells us about the general shape of the graph of the polynomial, which we’ll learn more about in future lessons.
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In an earlier activity, students graphed the function describing the relationship between the volume of a box made from a single piece of paper and the side length of the squares cut from the corners of the piece of paper. In this activity, students identify features of a similar polynomial function from the context and expression without a graph, so technology is not an appropriate tool. During the discussion, students use two equivalent forms of the expression to identify features of the graph, including intercepts.
Monitor for students who use these different strategies when determining the degree and leading term of the polynomial in the first question:
Rewrite the equation in standard form. Some students may complete the multiplication in different ways. For example, given three factors , , and , some students may multiply , while others choose .
Use diagrams to organize their work.
Multiply .
Launch
Representation: Develop Language and Symbols. Maintain a display of important terms and vocabulary. Invite students to suggest language or diagrams to include that will support their understanding of polynomial functions. Terms may include “horizontal and vertical intercepts,” “degree,” “leading term,” “factored form,” “standard form,” “constant,” “coefficient,” “equivalent,” and “volume.” Supports accessibility for: Conceptual Processing, Language
Activity
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Student Task Statement
We can make a box from a piece of paper that is 8.5 inches by 14 inches by cutting squares of side length from each corner and then folding up the sides. The volume , in cubic inches, of the box is a function of the side length , where .
Identify the degree and leading term of the polynomial. Explain or show your reasoning.
Without graphing, what can you say about the horizontal and vertical intercepts of the graph of ? Do these points make sense in this situation?
Student Response
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Building on Student Thinking
If students are unsure how to identify the degree or leading term of the polynomial written in factored form, consider asking:
“What did you notice and wonder about Equation A in the previous activity?”
“How could using the distributive property help you to identify the degree or leading term of the polynomial?”
Activity Synthesis
The goal of this discussion is for students to consider the advantages and disadvantages of polynomials written in factored form and standard form, and to see different ways to organize the use of the distributive property.
Display the function from the Task Statement for all to see. Invite previously selected students to share how they determined the degree and leading term of the polynomial. Here are some possible questions for discussion:
"Does it matter the order in which the factors are multiplied?" (No. Multiplying the factors in any order will have the same result.)
"How can drawing a diagram help to organize the work?" (A diagram can help make sure no terms were missed and help keep track of the signs of each term.)
If not brought up in students' explanations, display the function for all to see. Ask students which form of the polynomial, factored form or standard form, makes it easier to identify the following features:
Degree
Leading term
Constant
Horizontal intercept(s)
Vertical intercept
If time allows, display a set of scaled axes and add these features to the display. Then sketch in the cubic curve of at the conclusion as a small preview of the work students will do in future lessons investigating the general shape of polynomials with different degrees.
MLR7 Compare and Connect. After all strategies have been presented, lead a discussion comparing, contrasting, and connecting the different approaches. Ask, “What did the approaches have in common? How were they different?” “Why did the different approaches lead to the same outcome?” Advances: Representing, Conversing
Launch
Use the following examples and diagrams as necessary.
The first example is an expression with a completed diagram. Ask students where they see multiplication in the diagram and how they could use it to find an equivalent expression to the one given.
The second example has not been filled in. Ask students which expressions are being multiplied () and then to fill in the diagram.
The third example is the completed version. Ask students to find an equivalent expression to .
2
3
6
12
Representation: Internalize Comprehension. Use color coding and annotations to highlight connections between representations in a problem. For example, students can color-code like terms in their diagrams to reveal the terms they will combine before putting their answer in standard form. Some students may also benefit from writing in an exponent of 1 for any first-degree variables. This may help some students to more efficiently see structure while analyzing a sum and its corresponding addends, or a pair of factors and its corresponding product within the box diagram. Supports accessibility for: Visual-Spatial Processing
Activity
None
Student Task Statement
Use the distributive property to show that each pair of expressions is equivalent.
and
and
and
and
Student Response
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Building on Student Thinking
Activity Synthesis
Invite students to share their diagrams. If some students used methods other than diagrams to multiply the expressions, invite them to share and explain their method.
In this activity, students graph two related cubic polynomials to investigate the effect that multiplying by a constant has on input-output pairs of a function. Students are invited to use precise mathematical language as they describe how the features of the graphs are similar and different (MP6). They use two equivalent forms, standard and factored, to find properties of the functions, such as relative minimums and relative maximums.
In the digital version of the activity, students use an applet to visualize the effect of multiplying a polynomial by a constant, . The applet allows students to experiment with different values of . This activity works best when each student has access to devices that can run the Desmos applet, because students will benefit from seeing the relationship in a dynamic way. If students don’t have individual access, displaying the applet would be helpful during the Activity Synthesis.
Launch
Arrange students in groups of 2. Graphing technology is needed for every student. Display the two functions for all to see and ask students to predict how the graphs of each function will be the same or different. If necessary to save time, assign each partner either or to rewrite in the second question.
Activity Synthesis
The purpose of this discussion is for students to understand that multiplying by a constant causes all output values of a function to be times farther from the horizontal axis, which in turn means has no effect on the zeros of a function.
Begin the discussion by selecting 2–3 students to share how they rewrote and in standard form. If necessary, remind students how multiplying in a different order results in the same product.
Ask students, “How can you identify the constant term without multiplying out the entire expression?” (With an expression like , the constant term is 21 and comes from .)
If not brought up during the discussion, it is important to note that all three polynomials have the same degree and the same zeros, yet the three have distinct outputs for all other inputs. The idea that knowing the degree and zeros of a polynomial is insufficient for identifying a specific polynomial will be revisited in future lessons. If time allows, ask students to write an equation for a fourth function with the same degree and zeros as the first three.
MLR2 Collect and Display. Circulate to listen for and collect the language that students use as they multiply a function by a constant. On a visible display, record words and phrases, such as “same horizontal intercepts,” “closer to/farther from the horizontal axis,” or comparisons between the coordinates of the same input values on different graphs. Invite students to borrow language from the display as needed, and update it throughout the lesson. Advances: Conversing, Reading
Standards Alignment
Building On
Addressing
HSA-APR.B
Understand the relationship between zeros and factors of polynomials.
Understand that polynomials form a system analogous to the integers, namely, they are closed under the operations of addition, subtraction, and multiplication; add, subtract, and multiply polynomials.
Identify zeros of polynomials when suitable factorizations are available, and use the zeros to construct a rough graph of the function defined by the polynomial.
Identify the effect on the graph of replacing by , , , and for specific values of (both positive and negative); find the value of given the graphs. Experiment with cases and illustrate an explanation of the effects on the graph using technology. Include recognizing even and odd functions from their graphs and algebraic expressions for them.