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# Making Sense of Area Formulas for Regular Polygons and Circles

Lesson 3 of 14

## Objective: Students will be able to make sense of and explain area formulas for regular polygons and circles.

#### Warm-Up: Quadrilateral Area

*25 min*

At this point in the unit, I want to make sure students have a variety of strategies for thinking about and tackling area problems, especially problems that require them to persevere (**MP1**). Like all warm-ups in my class, I ask students to work quietly on their own. I circulate the room to see the strategies students are trying and to encourage good reasoning when I see it. When it appears that everyone has at least tried one strategy, I ask groups to begin discussing their methods, seeing what they agree and disagree with.

Many students tend to want to add a diagonal to the quadrilateral to find the area of each triangle—they are often stumped, however, as the base and height of each of these triangles is not clearly evident. While students often perceive this to be a dead end, I want to publicly validate the good reasoning here (the idea of finding a base and height) before we share out more successful methods. Ultimately, student volunteers share out, either decomposing the quadrilateral into four right triangles and a rectangle, or, building a rectangle around the quadrilateral and subtracting the extra areas gained by the four right triangles in the corners of the rectangle.

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In this group task, students will investigate regular polygons of different side lengths to ultimately generalize a formula for determining the area of any regular polygon.

I launch the task by having students construct a regular hexagon inscribed in a circle. Then, we review the mathematical terms **regular,** **congruent**, **isosceles**, and **apothem** as they see these terms in their construction. I then pass out the Regular Polygon Area task cards so students can carry out the investigation. Ultimately, I hope that each group will successfully arrive at the conjecture that **the area formula for any regular n-gon** is A=(1/2 as)n or A=1/2 aP where P=sn.

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After the group investigation, I facilitate a whole-class discussion in which I show students a circle that has been divided into 16 congruent sectors. I rearrange the sectors so that they form a parallelogram-like figure. I then give students about 2-3 minutes in their groups to see if they can make sense of why the area formula for a circle, which they will then share out with the whole class.

Because I want students to have a variety of ways to make sense of all of our area formulas, I show students the video below, which they watch silently, twice, before talking to a partner about their ideas. I then get the attention of the whole class and call on several students to explain how they made sense of the video and why the area formula for a circle is r^{2}.

**Source:** https://www.youtube.com/watch?v=whYqhpc6S6g (Accessed July 30, 2014)

#### Resources

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Like the last lesson, I want students to have time to explain the area formulas for regular polygons and circles in their own words. After students have written their explanations, they show them to me and get the Area Application, which they can begin in class.

#### Resources

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Although area measurement is not a major feature of the High School Geometry Common Core standards, it is important that students get opportunities to solve real-world area problems. In the Area Application Homework, students also get to explore how changing the dimensions of similar figures impacts their areas, which is often a challenging concept for students to grasp.

#### Resources

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- UNIT 1: Creating Classroom Culture to Develop the Math Practices
- UNIT 2: Introducing Geometry
- UNIT 3: Transformations
- UNIT 4: Discovering and Proving Angle Relationships
- UNIT 5: Constructions
- UNIT 6: Midterm Exam Review
- UNIT 7: Discovering and Proving Triangle Properties
- UNIT 8: Discovering and Proving Polygon Properties
- UNIT 9: Discovering and Proving Circles Properties
- UNIT 10: Geometric Measurement and Dimension
- UNIT 11: The Pythagorean Theorem
- UNIT 12: Triangle Similarity and Trigonometric Ratios
- UNIT 13: Final Exam Review

- LESSON 1: Sectors of Circles
- LESSON 2: Making Sense of Area Formulas for Triangles, Parallelograms, Trapezoids, and Kites
- LESSON 3: Making Sense of Area Formulas for Regular Polygons and Circles
- LESSON 4: Strategies for Decomposing 2-D Figures
- LESSON 5: Sector Area Application: The Grazing Goat
- LESSON 6: Surface Area and Area Differentiation
- LESSON 7: Extreme Couponing: Pizza Edition
- LESSON 8: Area "Quest"
- LESSON 9: Introduction to Volume: Origami Boxes
- LESSON 10: Origami Boxes Gallery Walk
- LESSON 11: Volume Formulas, Cavalieri's Principle, and 2-D Cross-Sections
- LESSON 12: Real World Volume Context Problems
- LESSON 13: Ratios of Similarity and 3D Solids Generated by Revolving 2D Figures
- LESSON 14: Volume "Quest"