Unit 5 - Defining Reflections

Learning Focus

Let's reflect some figures!

Jump Start

Unit 5 ● Lesson 2

Which one doesn’t belong?

Figure 1

Figure 4

Figure 3

Figure 2

Discuss

Unit 5 ● Lesson 2

What’s the Point: Reflections

Discuss

Unit 5 ● Lesson 2

What’s the Point: Reflections

Explore

With a partner discuss problems 1 – 4.

Unit 5 ● Lesson 2

Triangle in the Mirror

Kiran’s Diagram

Elena’s Diagram

Discuss

Kiran started reflecting triangle CDE across line m. So far, he knows the
image of D is D’ and the image of E is E’.

Unit 5 ● Lesson 2

Triangle in the Mirror

1. Annotate the diagram to show how he reflected

point D.

2. Use straightedge and compass moves to

determine the location of C’. Then lightly shade
in triangle C’D’E’.

3. Write a set of instructions for how to reflect any

point P across a given line l.

Discuss

Kiran started reflecting triangle CDE across line m. So far, he knows the image of D is D’ and the image of E is E’.

Unit 5 ● Lesson 2

Triangle in the Mirror

4. Elena found C’ incorrectly: Elena is

convinced that triangle C’D’E’ “looks
fine.” Explain to Elena why her C’ is not a
reflection of point C across line m.

Ready for More?

1. Using your response from question 2 (with the correct location of 𝐶′):

a. Draw the line 𝐶𝐶′.
b. Reflect triangle 𝐶′𝐷′𝐸′ across line 𝐶𝐶′.
c.

Label the image 𝐶"D"E“.

2. Find a single rigid motion that take 𝐶𝐷𝐸 to 𝐶"D"E“.

Unit 5 ● Lesson 2

Takeaways

Reflection is a rigid transformation that takes a point to another point
that is the same distance from the given line, is on the other side of the
given line, and so that the segment from the original point to the image is
perpendicular to the given line.

Reflect _(object)_ across line _(name)_.

Reflect A across line m.

Unit 5 ● Lesson 2

Exit Ticket

Lin tried to reflect triangle ABC across line t. She knows something went
wrong because the image isn’t congruent to the original figure.

1. What is one idea that Lin probably understands about reflections?
2. What is one idea that Lin doesn’t understand about reflections?
3. Reflect triangle ABC correctly.

Unit 5 ● Lesson 2