1. A High School Geometry Unit by Mary Doherty
Similar Triangles
A High School Geometry Unit
by Mary Doherty

2. Unit Objectives
Students will be able to determine if two triangles are similar.
Students will be able to find angle and side measures of similar triangles using congruence and proportions.
Students will apply the properties of similar triangles to solve problems.

3. NJ C.C.C.S. 4.2.Geometry and Measurement 5.4 Mathematical Processes
A.1:Use geometric models to represent real-world situations and objects and to solve problems using those models
E.1: Use techniques of indirect measurement to represent and solve problems (similar triangles)
5.4 Mathematical Processes
F.5: Use computer software to make and verify conjectures about geometric objects.

4. Day 1 Prerequisites: completed lessons on: Objectives
Ratios and proportions
Similar figures
Objectives
Students will use technology to explore the concept of similar triangles.
Students will make connections between similar figures and similar triangles and apply those connections to the properties of similar triangles.
Students will be able to identify similar triangles using one of three postulates and theorems.

5. Day 1 Warm-up Instructional Strategy Homework
students to read about similar figures at regentsprep.org to recall attributes (and initiate connections)
Instructional Strategy
students will work with a partner on SASinSchool InterActivity #890 (Triangles: Proving Similarity). Students to complete in-class worksheet. Teacher to serve as guide to facilitate achievement of objectives
Homework
SASinSchool follow-up worksheet

6. Day 2 Objectives Warm-up
Students will be able to identify similar triangles.
Students will apply the properties of similar triangles to solve problems.
Warm-up
Proving triangles congruent (to connect congruence short-cuts (i.e., SSS, SAS, AAS, and ASA) to similarity short-cuts (i.e., AA, SSS, SAS).

7. Day 2 Instructional Strategies Homework
Students to take notes on definitions of similar triangles and postulates and theorems used to prove triangles similar. Examples included in notes. (overhead projector)
Students to complete a brief problem set on determining whether two triangles are similar and, if so, stating the postulate or theorem used to prove this and writing a similarity statement.
Students to do three problems on finding angle or side measures by setting up proportions. Students to write solutions on blackboard.
Homework
worksheet from Teacher Resource workbook

8. Day 3
Objective
Students will use similar triangles and indirect measurement to measure the heights of large objects.
Warm-up
Students to view BrainPop on similar triangles (to introduce indirect measurement)

9. Day 3 Instructional Strategies Homework
Brief note-taking on concept of indirect measurement using similar triangles. Notes to include sketches and examples.
Students will work outside in groups of three to collect data (measuring shadows) to calculate the height of various large objects, such as trees, basketball backboards, flagpoles, etc.
After collecting data using the activity worksheet, students will return to the classroom and calculate the approximate height of these objects using similar triangles.
Groups to compare results.
Homework
worksheet of indirect measuring problems

10. Day 4
Objective
Students will use proportionality theorems to calculate lengths of sides in triangles.
Warm-up
Parallel line problems (parallel lines intersected by a transversal)

11. Day 4 Instructional Strategies Homework
Geometer’s Sketchpad Lab Parallel Lines in a Triangle. Students to construct sketches as directed and answer “discovery” questions.
Student note-taking on proportionality theorems with examples.
Guided practice solving problems applying theorems.
Homework
Textbook assignment to practice applying concepts

12. Day 5
Objective
Students will analyze how a ray bisecting an angle of a triangle divides the sides of the triangle proportionally.
Warm-up
Students to construct and label a triangle in Geometer’s Sketchpad with a ray bisecting an angle to prepare for investigative activity.

13. Day 5 Instructional Strategies Homework
Using the sketch constructed in the warm-up, students to measure sides of each the two triangles formed by the bisector. Students will then calculate ratios of sides to determine which corresponding parts are proportional.
Summary note-taking with examples
Independent practice
Homework
Worksheet from Teacher Resources

14. Unit Project Objectives
Students to construct and explore a “Golden Rectangle” and the Golden Ratio, a melding of art and history since ancient civilizations
Students to research history of Golden Ratio (and Golden Rectangles and Triangles) and applications in art, architecture and nature
Students to connect Sketchpad construction of the golden rectangle and golden spiral to its applications throughout the ages.

15. Unit Project Project Summary
Technology: construction of golden rectangle and golden spiral. Calculation of golden ratio.
Research to identify applications of golden rectangle, triangle, ratio, or spiral in history, art, architecture, nature or other area of student interest.
Communication: students to write a two-page essay to summarize findings and connect to construction.

16. Assessment Informal Formal Daily homework Student questions
Guided and independent practice
Student conjectures during investigative activities
Student blackboard work
Formal
Geometer’s Sketchpad lab reports and sketches
SasInSchool interactivity investigation
Indirect measuring activity summative results
Golden Rectangle project
Unit test