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# Prove exterior angles of triangle

Lesson 7 of 9

## Objective: SWBAT to prove a fact about exterior angles of a triangle.

*75 minutes*

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#### Play of the Day

*40 min*

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#### Huddle

*10 min*

Today is about getting more and more kids fluent in their ability to prove this fact, and so I push a few kids to come up to the board to present their proof, and to get evaluated by their peers. We use two simple questions to evaluate:

1. Is the logic correct?

2. Is the presentation clear and understandable?

To keep the proofs fresh after a few days, I put particular emphasis on students who chose different types of triangles to prove this one. How does using an equilateral triangle change this? How does using an isosceles triangle change this? What about using angles that are not at the "top" of the triangle?

We also engage in error analysis - sometimes students will draw incorrect transversals, or unnecessary lines. This pushes the students to be much more precise in their mathematical thinking.

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#### Homework

*15 min*

Tonight's Homework (7 - 7 HW97 Prove exterior angle of triangles) is to be completed at home and should students take approximately 15 minutes.

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- UNIT 1: FLUENCIES AND THE LANGUAGE OF ALGEBRA
- UNIT 2: SOLVING LINEAR EQUATIONS
- UNIT 3: INTRODUCTION TO FUNCTIONS
- UNIT 4: INTERPRETING AND COMPARING LINEAR FUNCTIONS
- UNIT 5: SYSTEMS OF LINEAR EQUATIONS
- UNIT 6: EXPONENTS AND SCIENTIFIC NOTATION
- UNIT 7: PARALLEL LINES, TRANSVERSALS, AND TRIANGLES
- UNIT 8: CONGRUENCE AND SIMILARITY THROUGH TRANSFORMATIONS
- UNIT 9: PATTERNS OF ASSOCIATION IN BIVARIATE DATA
- UNIT 10: SLOPE REVISITED
- UNIT 11: VOLUME OF CYLINDERS, CONES, AND SPHERES
- UNIT 12: POLYNOMIALS AND FACTORING
- UNIT 13: QUADRATIC FUNCTIONS

- LESSON 1: Special angle pairs
- LESSON 2: Transversals 1
- LESSON 3: Transversals 2
- LESSON 4: Practice transversals and blast from the past
- LESSON 5: Prove interior angles of a triangle using transversals
- LESSON 6: Prove interior angles of triangles 2
- LESSON 7: Prove exterior angles of triangle
- LESSON 8: Prove exterior angles of triangles 2
- LESSON 9: Arrange 3 copies to prove interior angles of triangles