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
In this activity, students are given the result of folding a shape along one or more lines of symmetry and asked to reason about the original shape. No lines of symmetry are specified, so students must consider all sides of a folded shape as a possible line of symmetry and visualize the missing half accordingly.
The first question offers opportunities to practice choosing tools strategically (MP5). Some students may wish to trace the half-shapes on patty paper, to make cutouts of them, or to use other tools or techniques to reason about the original shape. Provide access to the materials and tools they might need.
During the activity synthesis, discuss the different ways students approach the second question. Consider preparing cutouts of shapes A–F to facilitate the discussion. (The shapes are provided in the blackline master.)
Action and Expression: Internalize Executive Functions.
Invite students to plan a strategy, including the tools they will use, for the task. If time allows, invite students to share their plan with a partner before they begin.
Supports accessibility for: Conceptual Processing, Organization, Attention
Required Materials
Materials to Gather
Paper
Patty paper
Protractors
Rulers or straightedges
Scissors
Materials to Copy
Before and After
Launch
Groups of 2
Give a ruler or a straightedge to each student.
Provide access to protractors, patty paper, scrap paper, and scissors.
Activity
5 minutes: independent work time
2–3 minutes: partner discussion
Monitor for the different strategies students use to identify the original shape of the half-shapes (as noted in the activity narrative).
Student Facing
Mai has a piece of paper. She can get two different shapes by folding the paper along a line of symmetry. What is the shape of the paper before it was folded?
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Diego folded a piece of paper once along a line of symmetry and got this right triangle.
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Which shapes could the paper have before it was folded? Explain or show how you know.
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Student Response
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Activity Synthesis
Invite students to share their responses and strategies.
When discussing the second question, ask students why B, C, and E are not possible shapes of the original piece of paper even though there’s a line that breaks each figure into two right triangles that match the given triangle. (B and E have no line symmetry. C has lines of symmetry but not diagonally from corner to corner. If you fold there, the triangles would not be on top of one another.)


aligns: no