Lesson 17
Working with Rigid Transformations
17.1: Math Talk: From Here to There (5 minutes)
Warmup
The purpose of this warmup is to elicit strategies and understandings students have for using rigid motions on a pointbypoint basis without a grid. These understandings help students develop fluency and will be helpful later in this lesson when students will need to be able to put together rigid transformations that take one polygon to another again without making reference to a grid. While participating in this activity, students need to be precise in their word choice and use of language (MP6) because the rigid motions can only be defined in reference to labeled points.
Launch
Display the diagram and one problem at a time. Give students quiet think time for each problem and ask them to give a signal when they have an answer and a strategy. Keep all problems displayed throughout the talk. Follow with a wholeclass discussion.
Supports accessibility for: Memory; Organization
Student Facing
Segment \(CD\) is the perpendicular bisector of segment \(AB\). Find each transformation mentally.
A transformation that takes \(A\) to \(B\).
A transformation that takes \(B\) to \(A\).
A transformation that takes \(C\) to \(D\).
A transformation that takes \(D\) to \(C\).
Student Response
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Anticipated Misconceptions
Some students may think that \(C\) can be reflected over line \(AB\) onto \(D\). Ask them what we would need to know for that to work.
Activity Synthesis
Ask students to share their strategies for each problem. Record and display their responses for all to see. To involve more students in the conversation, consider asking:

“Who can restate \(\underline{\hspace{.5in}}\)’s reasoning in a different way?”

“Did anyone have the same strategy but would explain it differently?”

“Did anyone solve the problem in a different way?”

“Does anyone want to add on to \(\underline{\hspace{.5in}}\)’s strategy?”

“Do you agree or disagree? Why?”
If no student brings it up, ask students how a rotation may be used to take \(A\) to \(B\).
Design Principle(s): Optimize output (for explanation)
17.2: Card Sort: How Did This Get There? (20 minutes)
Activity
The purpose of this activity is to activate students’ prior knowledge about rigid and nonrigid transformations. Asking students to choose their own categories invites them to determine what characteristics might be important to notice. Students also begin to practice the skill of defining a rigid transformation that takes one figure onto the other without using a grid to estimate or define centers, angles, lines of reflection, or directed line segments.
Students may struggle to describe the location of the line of reflection. There is no need to address this during this activity as it will be covered in a subsequent activity.
Monitor for students who first translate to line up one pair of corresponding vertices, and then rotate for the card with triangle \(ABC\).
Launch
Arrange students in groups of 2. Distribute precut cards. Give students 3 minutes to sort and then pause for discussion. Ask a few groups to explain how they categorized their cards.
Invite students to write the transformations and follow with a wholeclass discussion.
Student Facing

Your teacher will give you a set of cards that show transformations of figures.

Sort the cards into categories of your choosing. Be prepared to explain the meaning of your categories.

Then sort the cards into categories in a different way. Be prepared to explain the meaning of your new categories.


For each card with a rigid transformation: write a sequence of rotations, translations, and reflections to get from the original figure to the image. Be precise.
Student Response
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Student Facing
Are you ready for more?
Diego observes that although it was often easier to use a sequence of reflections, rotations, and translations to describe the rigid transformations in the cards, each of them could be done with just a single reflection, rotation, or translation. However, Priya draws her own card, shown, which she claims can not be done as a single reflection, rotation, or translation.
 For each rigid transformation from the card sort, write the transformation as a single reflection, rotation, or translation.
 Justify why Priya’s transformation cannot be written as a single reflection, rotation, or translation.
Student Response
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Activity Synthesis
One purpose of discussion is to reemphasize that translations, rotations, and reflections are rigid transformations, which maintain the size and shape of polygons while other transformations do not. Remind students that figures are called congruent when there is a sequence of rigid transformations that takes one figure onto another.
The second purpose of discussion is to help students identify what is hard about defining rigid transformations off the grid. If students struggle to define rigid motions precisely, there is an optional lesson on pointbypoint transformations to use in addition to this discussion.
Consider the card with triangles \(ABC\) and \(A'B'C'\). If any student first translated to line up one pair of corresponding vertices, and then rotated, call on that student to share their method. If not, discuss the difficulty of estimating where to place the center of rotation so that a single rotation will definitely line up all three points. Demonstrate for students how you can translate by the directed line segment from \(C\) to \(C'\), and then rotate by the angle formed at vertex \(C\).
If no students used a pointbypoint method, encourage them to take another look at the card with \(VWXYZ\) and try to use a pointbypoint method to define rigid motions that take the original figure onto the image without estimating the locations of any new lines or points.
Wait on a detailed discussion of the transformation of the card with \(RSTU\), as it will be the focus of the next activity.
Design Principle(s): Maximize metaawareness
17.3: Reflecting on Reflection (10 minutes)
Activity
In this activity, students determine whether a reflection can take one figure to another and how to find the precise line of reflection for disjoint figures.
Making dynamic geometry software available gives students an opportunity to choose appropriate tools strategically (MP5).
Launch
Supports accessibility for: Language; Socialemotional skills
Student Facing
Diego says, “I see why a reflection could take \(RSTU\) to \(R'S'T'U'\), but I’m not sure where the line of reflection is. I’ll just guess.”
 How could Diego see that a reflection could work without knowing where the line of reflection is?
 How could Diego find an exact line of reflection that would work?
Student Response
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Launch
Tell students to refer to the card from the previous activity with figure \(RSTU\).
Supports accessibility for: Language; Socialemotional skills
Student Facing
Diego says, “I see why a reflection could take \(RSTU\) to \(R'S'T'U'\), but I’m not sure where the line of reflection is. I’ll just guess.”
 How could Diego see that a reflection could work without knowing where the line of reflection is?
 How could Diego find an exact line of reflection that would work?
Student Response
For access, consult one of our IM Certified Partners.
Activity Synthesis
The purpose of this discussion is to reinforce using defintions to justify a response. Invite a few students to share their responses with the class. Highlight any improvements over previous justifications such as good use of a welllabeled diagram.
Lesson Synthesis
Lesson Synthesis
Display two congruent triangles for all to see:
Ask students:
 Which triangle is the original and which is the image? (The triangle on the right is the original and the triangle on the left is the image. The prime notation on the labels of the vertices gives it away.)
 What would be a sequence of transformations that would take triangle \(ABC\) onto triangle \(A’B’C’\)? Explain how you know the image of \(A\) coincides with \(A’\). (Translate \(B\) onto \(B’\), and then rotate until the image of \(A\) lands on \(A’\). The transformations keep the distances the same, so the image of \(A\) is on ray \(B’A’\) at a distance of \(B’A’\). That means the image of \(A\) has to land on \(A’\).)
 Would that sequence work just for this one pair of congruent triangles, or would it work for other pairs of congruent triangles, too? (This sequence will work for any pair of congruent triangles, so long as there isn’t a reflection involved.)
Display another image for all to see:
Ask students:
 What is different about this from the previous case? (The previous case does not involve a reflection.)
 What happens when we try the same sequence of transformations from last time? (If we translate \(B\) onto \(B’\) and then rotate until the image of \(A\) lands on \(A’\), then the triangles will share side \(A’B’\), but the image of \(C\) will be on the opposite side of line \(A’B’\) as \(C’\).)
 What do we need to add to the sequence to take triangle \(ABC\) onto triangle \(A’B’C’\)? (Performing a reflection across the line \(A’B’\) will take the image of \(C\) onto \(C’\).)
Tell students that defining these sequences and looking at the results is enough to conjecture that the sequences take one figure onto another. To be convincing, it is necessary to be able to explain why each vertex lands exactly where we think it should land. Explaining why each point lands exactly where it should is something they will work on over time.
17.4: Cooldown  Get This There (5 minutes)
CoolDown
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Student Lesson Summary
Student Facing
If 2 figures are congruent, we can always find a rigid transformation that takes one onto the other.
Look at congruent figures \(ABC\) and \(DEF\). It looks like \(DEF\) might be a reflection and translation of \(ABC\). But is there a way to describe a sequence of transformations without guessing where the line of reflection might be?
Our goal is to take the image of \(E\) onto \(B\). Then we want to take the image of \(D\) onto \(A\) without moving \(E\) and \(B\). Finally, we need to take the image of \(F\) onto \(C\) without moving any of the matching points.
We can start with translation: Translate triangle \(DEF\) by the directed line segment from \(E\) to \(B\).
Now, a pair of corresponding points coincides. Is there a transformation we could use to take \(D'\) onto \(A\) that leaves \(B\) and \(E'\) in place? Rotations have a fixed point, so rotate triangle \(D'E'F'\) by angle \(D'BA\) using point \(B\) as the center.
Now, 2 pairs of corresponding points coincide. Reflecting across line \(AB\) will take \(D''E''F''\) onto \(ABC\), which is what we were trying to do. We know \(D''\) and \(E''\) won’t move, since points on the line of reflection don't move. How do we know \(F''\) will end up on \(C\)? Since the triangles are congruent, \(F''\)and \(C\) are the same distance from the line of reflection.
It is always possible to describe transformations using existing points, angles, and segments. It could take an extra step, but we can be confident transformations work if we don't guess where the line of reflection or center of rotation might be.