Lesson 4

The purpose of this warm-up is to elicit the idea of mirror images that match exactly, which will be useful when students generate a definition for symmetry in a later activity. Students may notice and wonder about possible real-life objects the block structure represents, but focus the discussion on describing how the two halves are reflections of each other.

Elicit the term “symmetry” or language that describes attributes of line-symmetric figures.

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

• “Discuss your thinking with your partner.”
• 1 minute: partner discussion
• Share and record responses.

• Highlight the language students used to describe the symmetry in the design such as:
  • mirror images
  • the same parts on both sides of the image
• “How might we check to see if the part on the left and that on the right are the same?” (We could place each half of the image on top of the other to see if they match.)

This activity uses the idea of folding to introduce students to line symmetry. Students analyze examples of figures that have a line of symmetry and those that don’t, and use their observations to formulate a definition of line of symmetry, which they then refine with their peers (MP6). Students later identify and draw lines of symmetry in various figures.

Some students may wish to use tools to help them define and find lines of symmetry. Provide access to patty paper, rulers, protractors, scissors, and copies of the figures (provided in the blackline master), if requested.

This activity uses MLR1 Stronger and Clearer Each Time. Advances: Reading, Writing.

• Make copies of the set of figures in the second question available for cutting and for demonstration during the lesson synthesis.

• Groups of 2–4
• Give a ruler or straightedge to each student.
• Provide access to patty paper, protractors, scissors, and copies of the shapes in the second question.
• Display or sketch these parallelograms.

  

  

  “How are the two figures alike? How are they different?”

• 1 minute: quiet think time
• Discuss responses. (Students may say:
  • Both parallelograms show a dashed line through opposite corners.
  • The dashed line creates two identical triangles.
  • The triangles in the first shape would match up exactly if the shape is folded along the line, but not so with those in the second shape.)

• 3 minutes: independent work time on the first question

MLR1 Stronger and Clearer Each Time

• “Share your response to ‘a line of symmetry is…’ with your partner. Take turns being the speaker and the listener. If you are the speaker, share your ideas and writing so far. If you are the listener, ask questions and give feedback to help your partner improve their work.”
• 2 minutes: structured partner discussion 
• Repeat with 2–3 different partners.
• “Revise your initial draft based on the feedback you got from your partners.”
• 2–3 minutes: independent work time
• 3 minutes: independent work time on the remaining questions

If students identify a line of symmetry that would not result in two identical halves of the original figure, consider asking:

• “How did you decide where to draw your line? How do you know that it's a line of symmetry?”
• “How can you use a tool to prove that your line is a line of symmetry?”

• Ask groups to share their definitions of line of symmetry and record key phrases. Invite the class to comment.
• Discuss responses to the last two questions. Ask students to share the lines of symmetry they drew.
• “How might we check that the line we drew is a line of symmetry? How can we tell if the two halves that appear to be identical are indeed the same?” (Some ways:
  • Cut out the figure and fold it along the line we drew.
  • Trace one half of the figure on patty paper and turn it over to see if it matches the other half.
  • Measure the segments and angles.)
• “If a figure can be folded along a line to create two parts that are a mirror reflection of one another and would match up exactly, we say that the figure has symmetry or has line symmetry.”
• “The line that splits a figure into two mirroring and matching parts is called a line of symmetry.”

In this activity, students practice identifying two-dimensional figures with line symmetry. They sort a set of figures based on the number of lines of symmetry that the figures have.

Continue to provide access to patty paper, rulers, and protractors. Students who use these tools to show that a shape has or does not have a line of symmetry use tools strategically (MP5). Consider allowing students to fold the cards, if needed.

• Sort the shape cards from the previous lessons into three groups of 12 cards (A–L, M–X, and Y–JJ).

• Groups of 2
• Give each group a set of 12 shape cards (A–L, M–X, or Y–JJ).
• Provide access to patty paper, rulers, and protractors.

• “Work together with a partner to sort the figures by the number of lines of symmetry.”
• 5 minutes: group work time

If students draw fewer or more lines of symmetry than the figure actually has, consider asking:

• “How did you decide where to draw your line(s)?”
• “How can you prove that each line is a line of symmetry?”
• “Have you found all the lines of symmetry for this figure? How do you know?”

• “How did you decide if a figure had a line of symmetry? What did you look for?” (Look for matching parts on opposite sides of a figure—parts that have the same size and mirror each other.)
• “Were there figures that you could immediately tell had no lines of symmetry? What was it about the figures that made it clear?” (The parts are different from one another. No folding lines could make two parts mirror each other.)

This optional activity gives students an opportunity to reason about lines of symmetry. Students see that some shapes can be folded in half again and again. Some students may notice that certain shapes, like a square, can be divided into two equal halves in perpetuity (MP8). Each new half created by a line of symmetry will continue to be line-symmetric.

• Groups of 2
• Provide access to patty paper and rulers or straightedges.

• 4–5 minutes: independent work time
• 1–2 minutes: group discussion

• “A, B, and C have more than one line of symmetry. Does the folding line we start with affect how many times the shape could be folded before there is no more line symmetry?” (In these shapes, no, the starting fold doesn’t determine how many folds can be made afterward.)
• 1 minute: quiet think time
• “How can you tell when a folded shape no longer has line symmetry?” (No segments are the same length and no angles are the same size.)
• “What new ideas about lines of symmetry did you learn?” (Sample responses:
  • Folding a shape along a line of symmetry may create new shapes that also have lines of symmetry. These lines may create more shapes with symmetry.
  • For some shapes, this process could go on even if we cannot physically fold the paper any longer.)