Transformations and Angle Relationships

Lesson 15

Math

Unit 3

8th Grade

Lesson 15 of 22

Objective


Find missing side lengths in similar figures. Find scale factor between similar figures.

Common Core Standards


Core Standards

  • 8.G.A.4 — Understand that a two-dimensional figure is similar to another if the second can be obtained from the first by a sequence of rotations, reflections, translations, and dilations; given two similar two-dimensional figures, describe a sequence that exhibits the similarity between them.

Foundational Standards

  • 7.G.A.1
  • 7.RP.A.2
  • 7.RP.A.3

Criteria for Success


  1. Use ratio reasoning between corresponding sides in similar figures to determine missing measurements. 
  2. Find the scale factor between similar figures and use it to find missing measurements.
  3. Determine if two figures are similar by testing for a scale factor between the side lengths.

Tips for Teachers


In this lesson, students apply the concept of scale factor to determine if shapes are similar with proportional corresponding sides.

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Anchor Problems

25-30 minutes


Problem 1

Two triangles are shown below. The measure of angle $$A$$ is equal to the measure of angle $$D$$. (Figures are not drawn to scale.)

a.   Explain, using transformations, how you know the two triangles are similar. 

b.   Find the value for the side length marked $$x$$.

Guiding Questions

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Student Response

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Problem 2

Are triangles $${{ABC}}$$ and $${{EFG}}$$ similar? Explain your answer. 

Sketch a triangle that is similar to either triangle $${{ABC}}$$ or triangle $${{EFG}}$$. Label the side lengths. 

Guiding Questions

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Student Response

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Problem 3

Triangle $${{ABC}}$$ is similar to triangle $${{ADE}}$$. The measure of $${\overline{AB}}$$ is approximately $${3.16}$$ units.

a.   Find the scale factor from triangle $${{ABC}}$$ to $${{ADE}}$$.

b.   Find the approximate measure of $${\overline{AD}}$$.

Guiding Questions

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Student Response

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Problem Set

15-20 minutes


Give your students more opportunities to practice the skills in this lesson with a downloadable problem set aligned to the daily objective.

Target Task

5-10 minutes


Problem 1

Three parallelograms are shown below. Which parallelogram, either $$B$$ or $$C$$, is similar to parallelogram $$A$$? Justify your response.

Student Response

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Problem 2

Triangles $$QRS$$ and $$JKL$$ are similar. Use the diagrams below to answer the following questions.

a.   What is the length of line segment $$SR$$? Show or explain your reasoning. 

b.   What is the length of line segment $$JK$$? Show or explain your reasoning.

Student Response

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Additional Practice


The following resources include problems and activities aligned to the objective of the lesson that can be used for additional practice or to create your own problem set.

Next

Use properties of similar triangles to model and solve real-world problems.

Lesson 16
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Lesson Map

A7CB09C2-D12F-4F55-80DB-37298FF0A765

Topic A: Congruence and Rigid Transformations

Topic B: Similarity and Dilations

Topic C: Angle Relationships

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