standard_description: Give an informal argument using Cavalieri’s principle for the formulas for the volume of a sphere and other solid figures.
problem_activity: Activity
During this activity students are asked to compute the perimeter of inscribed regular polygons given only the radius. They will need to figure out that drawing in the altitude will form a right triangle. Drawing this auxiliary segment shows students recognize the important structure of right triangles to calculate missing information (MP7). In this activity students are building towards an equation to approximate the value of
\(\pi\)
which they will complete in the next lesson.
Launch
Provide the definitions of a regular polygon (a polygon where all of the sides are congruent and all the angles are congruent), pentagon (5-sided polygon), and decagon (10-sided polygon) as needed.
Representation: Develop Language and Symbols.
Create a display of important terms and vocabulary. During the launch, take time to review terms that students will need to access for this activity. Invite students to suggest language or diagrams to include that will support their understanding of: regular polygons, pentagons, and decagons. Include definitions as well as diagrams for students to refer to throughout the lesson.
Supports accessibility for: Conceptual processing; Language
Student Facing
Here is a square inscribed in a circle with radius 1 meter. What is the perimeter of the square? Explain or show your reasoning.
[IMG]
What is the perimeter of a regular pentagon inscribed in a circle with radius 1 meter? Explain or show your reasoning.
What is the perimeter of a regular decagon inscribed in a circle with radius 1 meter? Explain or show your reasoning.
What is happening to the perimeter as the number of sides increases?
Student Response
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Student Facing
Are you ready for more?
Here is a diagram of a square inscribed in a circle and another circle inscribed in the same square.
[IMG]
How much shorter is the perimeter of the small circle than the perimeter of the large circle?
If the square was replaced with a regular polygon with more sides, would your previous answer be larger, smaller, or the same? Explain or show your reasoning.
Student Response
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for free access to Extension Student Response.
Anticipated Misconceptions
If students are spending too long attempting to draw the pentagon or decagon, provide them with a circle that has 10 equally spaced points.
If students are stuck after sketching the diagram ask them what helpful auxiliary lines they could draw. (Connect the center to the other vertices to make triangles. The altitude would create right triangles so I can use trigonometry.)
[IMG]
Activity Synthesis
Invite students to share their strategies. Focus on students who repeated the same strategy for multiple questions.
Use the applet to demonstrate what happens with the perimeter of the inscribed polygon as the number of sides increases.
Ask students what they noticed. Connect the generalizations they make with the noticing and wondering they did in the warm-up. If not mentioned by students, ask students what the circumference of the circle is. (
\(2\pi\)
) Build anticipation for calculating the value of
\(\pi\)
in the next lesson.
If students are not using workbooks tell them to be extra careful to put this work in a safe place because they will use it in the next lesson.
Writing, Listening, Conversing: MLR1 Stronger and Clearer Each Time.
Use this routine to help students improve their writing, by providing them with multiple opportunities to clarify their explanations through conversation. Give students time to meet with 2–3 partners to share and get feedback on their work. Display prompts for feedback that students can use to help their partner strengthen and clarify their ideas. For example, "Your explanation tells me . . .", "Can you say more about why you . . . ?", and "A detail (or word) you could add is _____, because . . . ." Give students with 3–4 minutes to revise their initial draft based on feedback from their peers.
Design Principle(s): Optimize output (for explanation); Cultivate conversation


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