Transformations and Angle Relationships

Students investigate congruence and similarity by studying transformations of figures in the coordinate plane, and apply these transformations to discover new angle relationships.

Unit Summary

In Unit 3, eighth-grade students bridge their understanding of geometry from middle school concepts to high school concepts. Until now, standards in the geometry domain have been either supporting or additional to the major standards. In eighth grade, geometry standards, especially the standards highlighted in this unit, are part of the major work of the grade and play a critical role in setting students up well for success in high school.

In this unit, students begin their work with transformations using patty paper (transparency paper) to experiment with, manipulate, and verify hypotheses around how shapes move under different transformations (MP.5). They use precision in their descriptions of transformations and in their justifications for why two figures may be similar or congruent to each other (MP.6). Students then apply their understanding of transformations to discover new angle relationships in parallel line diagrams and triangles.  

Prior to eighth grade, students developed their understanding of geometric figures and learned how to draw them, calculate measurements, and model real-world situations. In seventh grade, students were introduced to the concept of scaling through scale drawings, and they solved for various measurements using proportional reasoning. Students will draw on these prior skills when they investigate dilations and similar triangles. 

In high school geometry, students spend significant time studying congruence and similarity in-depth. They build off of the informal proofs and reasoning developed in eighth grade to hone their definitions of transformations, prove geometric theorems, and derive trigonometric ratios.

Pacing: 26 instructional days (22 lessons, 3 flex days, 1 assessment day)

For guidance on adjusting the pacing for the 2021-2022 school year, see our 8th Grade Scope and Sequence Recommended Adjustments.

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This assessment accompanies Unit 3 and should be given on the suggested assessment day or after completing the unit.

Unit Prep

Intellectual Prep


Internalization of Standards via the Post-Unit Assessment

  • Take the Post-Unit Assessment. Annotate for: 
    • Standards that each question aligns to
    • Strategies and representations used in daily lessons
    • Relationship to Essential Understandings of unit 
    • Lesson(s) that Assessment points to

Internalization of Trajectory of Unit

  • Read and annotate the Unit Summary.
  • Notice the progression of concepts through the unit using the Lesson Map.
  • Do all Target Tasks. Annotate the Target Tasks for: 
    • Essential Understandings
    • Connection to Post-Unit Assessment questions
  • Identify key opportunities to engage students in academic discourse. Read through our Teacher Tool on Academic Discourse and refer back to it throughout the unit.

Unit-Specific Intellectual Prep

Essential Understandings


  • Two figures are congruent to each other if there exists a sequence of rigid transformations that will map one figure onto the other.
  • Two figures are similar to each other if there exists a sequence of dilations and rigid transformations that will map one figure onto the other. 
  • Certain properties are preserved under rigid transformations (such as angle measurement, line segment length, and parallel line relationships).
  • Angle relationships exist in polygons, intersecting lines, and parallel lines that can be used to determine various angle measurements.



  • 180° Protractor (1 per student)
  • Optional: Calculators (1 per student)
  • Graph Paper (2-3 sheets per student)
  • Scissors (1 per small group)
  • Ruler (1 per student)
  • Optional: Tape (1 per small group)
  • Patty paper (transparency paper) (several sheets per student)

To see more information about the materials in this unit, view the Unit Materials Overview.



alternate interior and exterior angles

corresponding angles




rigid transformation


congruent/ congruence


scale factor

vertical angles

To see all the vocabulary for this course, view our 8th Grade Vocabulary Glossary.

Lesson Map

Topic A: Congruence and Rigid Transformations






Describe and perform rotations between congruent figures.




Describe multiple rigid transformations using coordinate points.

Topic B: Similarity and Dilations



Define a dilation as a non-rigid transformation, and understand the impact of scale factor.



Describe and perform dilations.



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



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

Topic C: Angle Relationships



Solve for missing angle measures in parallel line diagrams using equations.



Define and use the interior angle sum theorem for triangles.



Define and use the exterior angle theorem for triangles.



Define and use the angle-angle criterion for similar triangles.

Common Core Standards

Key: Major Cluster Supporting Cluster Additional Cluster

Core Standards


  • 8.G.A.1 — Verify experimentally the properties of rotations, reflections, and translations:

  • 8.G.A.1.A — Lines are taken to lines, and line segments to line segments of the same length.

  • 8.G.A.1.B — Angles are taken to angles of the same measure.

  • 8.G.A.1.C — Parallel lines are taken to parallel lines.

  • 8.G.A.2 — Understand that a two-dimensional figure is congruent to another if the second can be obtained from the first by a sequence of rotations, reflections, and translations; given two congruent figures, describe a sequence that exhibits the congruence between them.

  • 8.G.A.3 — Describe the effect of dilations, translations, rotations, and reflections on two-dimensional figures using coordinates.

  • 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.

  • 8.G.A.5 — Use informal arguments to establish facts about the angle sum and exterior angle of triangles, about the angles created when parallel lines are cut by a transversal, and the angle-angle criterion for similarity of triangles. For example, arrange three copies of the same triangle so that the sum of the three angles appears to form a line, and give an argument in terms of transversals why this is so.

Foundational Standards


Expressions and Equations
  • 8.EE.C.7

  • 7.G.A.1

  • 7.G.A.2

  • 7.G.B.5

Measurement and Data
  • 4.MD.C.6

Ratios and Proportional Relationships
  • 7.RP.A.2

  • 7.RP.A.3

Future Standards


  • G.CO.A.2

  • G.CO.A.3

  • G.CO.A.4

  • G.CO.A.5

  • G.CO.B.6

  • G.CO.B.7

  • G.CO.B.8

  • G.CO.C.10

  • G.CO.C.9

Similarity, Right Triangles, and Trigonometry
  • G.SRT.A.1

  • G.SRT.A.2

  • G.SRT.A.3

  • G.SRT.B.4

  • G.SRT.B.5

Standards for Mathematical Practice

  • CCSS.MATH.PRACTICE.MP1 — Make sense of problems and persevere in solving them.

  • CCSS.MATH.PRACTICE.MP2 — Reason abstractly and quantitatively.

  • CCSS.MATH.PRACTICE.MP3 — Construct viable arguments and critique the reasoning of others.

  • CCSS.MATH.PRACTICE.MP4 — Model with mathematics.

  • CCSS.MATH.PRACTICE.MP5 — Use appropriate tools strategically.

  • CCSS.MATH.PRACTICE.MP6 — Attend to precision.

  • CCSS.MATH.PRACTICE.MP7 — Look for and make use of structure.

  • CCSS.MATH.PRACTICE.MP8 — Look for and express regularity in repeated reasoning.