Lesson 10

This warm-up encourages students to look for and make use of structure in an image to identify the lines of symmetry it has (MP7). Students could try to find all the segments or angles that are the same size as a way to identify lines of symmetry, but keeping track of all the pieces can be rather impractical. Instead, students could pay attention to the composition of the image— a five-sided figure on the outside and a five-point star on the inside, where all of the five parts are the same—and use that insight to determine the number of lines of symmetry.

• Groups of 2
• “How many do you see? How do you see them?”
• Display the image.
• 1 minute: quiet think time

• Display the image.
• “Discuss your thinking with your partner.”
• 1 minute: partner discussion
• Record responses.

• Invite students to share how they identified the lines of symmetry. Display the original image for students to mark up and use in their explanation.
• Some students may have double-counted and say that there are 10 lines of symmetry. Invite others to respond to that claim.
• Consider asking:
  • “Who can restate the way _____ saw the lines of symmetry in different words?”
  • “Did anyone see the lines of symmetry the same way but would explain it differently?”
  • “Does anyone want to add an observation to the way _____ saw the lines of symmetry?”

This activity serves several goals. Students continue to practice visualizing and drawing a complete shape given a line of symmetry and one half of the shape. As they do so, they practice drawing angles of certain measurements. Students also use symmetry to reason about unknown angle measurements in two-dimensional figures.

For the drawing portion of the activity, assign a different shape for each group member to start with and ask students to draw as precisely as possible (MP6). Provide access to protractors and patty paper (MP5). Most students are likely to find Andre’s shape most challenging to draw. Differentiate the starting drawing for each student as needed.

• Read the opening paragraph of the activity statement as a class. Display the four images. Clarify the context as needed before students begin the activity.
• Groups of 4
• Ask each group member to start with the drawing for a different student (one member starts with Noah’s, another with Clare’s, and so on), but try to complete at least 2 of the 4 drawings.
• Give a protractor and a ruler to each student.
• Provide access to patty paper, scrap paper, and scissors.

• 7–8 minutes: independent work time
• 3–4 minutes: group discussion
• Monitor for the different ways students use tools to draw precisely.
• Pause for a brief class discussion. Invite students to share their completed drawings before students proceed to the second question.

If students identify some but not all of the interior angles of the unfolded paper, consider asking:

• “How did you find the angle measurements of the unfolded paper without measuring?”
• “Have you found all the angles within the shape of the unfolded paper? How do you know?”
• “How can you use the angles in the original shapes to find the new angles in the shape of the unfolded paper?”

• Display the drawings that students completed and select students to share how they found the angles inside each shape.
• Highlight that lines of symmetry can be used to identify angles that have the same size as a given angle, or angles that are twice the size of a given angle.

In this activity, students apply their understanding of symmetry and knowledge of angles to find angle measurements in a more complex line-symmetric figure. Students use what they know about the measurement of a straight angle and the measurement of a full rotation of a ray around a point to find the unknown angles. As needed, review the measurements of benchmark angles (\(90^\circ\), \(180^\circ\), \(360^\circ\)) before the activity.

• Groups of 2
• Display the image.
• “What do you notice? What do you wonder?”
• 1 minute: quiet think time
• 1 minute: discuss observations and questions

• 5 minutes: independent work time
• 3 minutes: partner discussion

Students may find the measurement of some angles using the figure's symmetry (angles b, c, and f), but not of angles that require reasoning about a full turn or half turn around a point (angles a, d, and e). Consider asking:

• “What did you do to find the measurement of these angles? Will that strategy work for the other angles? Why or why not?”
• “What angles share the same vertex and rays as the angle that is unknown? How could you use them to find the the unknown angle?”
• “What do you know about the measure of (a full turn or a half turn) around a point? How could that help you find an unknown angle?”

• Invite students to share their responses and reasoning.
• Highlight that:
  • Angles \(a\) and \(d\) are each part of a straight angle, so each can be found by subtracting the adjacent angle measure from 180.
  • Angle \(e\) and the two angles one either side of it make a full turn or \(360^\circ\).
• Consider asking: “Which other angle measurements in or around the fish diagram can you find?” If time permits, encourage students to find as many as they can.