standard_description: Derive using similarity the fact that the length of the arc intercepted by an angle is proportional to the radius, and define the radian measure of the angle as the constant of proportionality; derive the formula for the area of a sector.
problem_activity: Narrative
The purpose of this activity is for students to practice finding the perimeter of shapes that have labeled side lengths. The synthesis focuses on methods students have for efficiently finding the perimeter of shapes with some or all side lengths having equal length. As students discuss and justify their decisions, they share a mathematical claim and the thinking behind it (MP3).
Monitor and select students who find the perimeter of the hexagon by:
adding the individual side lengths around the shape
adding the two 8–inch side lengths together and the four 4–inch side lengths together and then adding those sums together
multiplying like side lengths, then adding, such as
\(2 \times 8\)
for the long sides and
\(4 \times 4\)
for the short sides and then adding those products together
using symmetry to split the shape in half horizontally and adding
\(4 + 8 + 4 = 16\)
for the top half of the shape and then doubling that for the sides on the bottom half of the shape
MLR8 Discussion Supports.
Synthesis: Provide students with the opportunity to rehearse what they will say with a partner before they share with the whole class.
Advances: Speaking
Launch
Groups of 2
Display the image.
“What do you notice? What do you wonder?” (Students may notice: One rectangle has numbers on the sides. One rectangle has tick marks on the sides. The rectangles are the same size. Students may wonder: Why are the sides of the rectangles marked differently? Could we find the distance around the rectangle with the numbers on the sides?)
1 minute: quiet think time
1 minute: partner discussion
Share and record responses.
Activity
“Work with your partner to find the perimeter of each shape.”
5–7 minutes: partner work time
As students work, consider asking:
“How does having sides of the same length help us find the perimeter?”
“Can you multiply to find the perimeter?”
Student Facing
What do you notice? What you wonder?
[IMG]
Find the perimeter of each shape. Explain or show your reasoning.
[IMG]
[IMG]
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Student Response
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Activity Synthesis
Ask previously identified students to share their strategies for finding the perimeter of the hexagon. Arrange the presentations in the order listed in the activity narrative.
Give students a chance to ask questions about each strategy as it is shared.
“How does having sides of the same length help us find the perimeter?”
Consider asking:
“Why does this strategy work with this shape?”
“Did anyone else find the perimeter of this shape in a different way?”
“Was it easier to find the perimeter of some shapes in this activity than others? Why?” (Yes, some of the shapes had several sides that are the same length, so we could multiply. In a rectangle, we can add two sides and then double the result to find the whole perimeter.)


aligns: no