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# Similar Triangle Practice

Lesson 9 of 10

## Objective: SWBAT determine when triangles are similar.

#### Do Now

*5 min*

In the Do Now, students are given two triangles, each with the measure of one angle identified. They are asked what the measures of each angle of the triangles would have to be in order for the triangles to be similar. Based on theorems from previous lessons, students know that two angles of a triangle must be congruent in order for two triangles to be similar. They must use the measures from both triangle to prove similarity. If students use different measures, the triangles won't be congruent.

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#### Mini-Lesson

*5 min*

For the Mini-Lesson, we go over a problem before students work on the activity. Students work together to find the length of a missing side in a triangle. This provides a check-in with the students to ensure they understand how to find the measure of missing sides in a triangle using criteria for similarity.

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#### Activity

*20 min*

In the Activity students decide if two triangles are similar given measurements of sides or angles (**G.SRT.2, G.SRT.5)**. They justify their answer using calculations. On the worksheet, I have only given the students one diagram for five questions. Because of this, some students may have difficulty identifying corresponding parts of the triangles using only the names of the parts. To help them, I have the students color corresponding parts to help identify them.

Students work independently on the activity. After about 12 minutes, students share their work with the students at their group. If there are any discrepancies, students must decide who is correct based on their justifications and understanding of the criteria for similar triangles.

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#### Summary

*15 min*

At the end of today's lesson, students will take a quiz. The quiz has questions about dilations, congruence, and similarity. I use the results of the quiz to assess how well the students understand triangle similarity. If necessary, I provide the students with extra practice proving triangles are similar. I use this quiz as more of a formative assessment and don't give students a grade.

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- UNIT 1: Preparing for the Geometry Course
- UNIT 2: Geometric Constructions
- UNIT 3: Transformational Geometry
- UNIT 4: Rigid Motions
- UNIT 5: Fall Interim Assessment: Geometry Intro, Constructions and Rigid Motion
- UNIT 6: Introduction to Geometric Proofs
- UNIT 7: Proofs about Triangles
- UNIT 8: Common Core Geometry Midcourse Assessment
- UNIT 9: Proofs about Parallelograms
- UNIT 10: Similarity in Triangles
- UNIT 11: Geometric Trigonometry
- UNIT 12: The Third Dimension
- UNIT 13: Geometric Modeling
- UNIT 14: Final Assessment

- LESSON 1: Scale Factor
- LESSON 2: Dilations on the Coordinate Plane, Center (0, 0)
- LESSON 3: Dilations using Geometer's Sketchpad
- LESSON 4: Dilations on the Coordinate Plane, Center (h, k)
- LESSON 5: Properties of Dilations Extension Lesson
- LESSON 6: Similar Triangles using Geometer's Sketchpad
- LESSON 7: Finding Missing Sides of Similar Triangles
- LESSON 8: Angle-Angle Similarity Postulate
- LESSON 9: Similar Triangle Practice
- LESSON 10: Similar Triangles and the Flatiron Building