## Reflection: Staircase of Complexity Prove exterior angles of triangle - Section 2: Play of the Day

This year, rather than prove the exterior angle theorem using transversals, I thought it might be MUCH more intuitive to prove the exterior angle theorem using supplementary angles, and lead students through a series of problems that helps to derive this.

I have to say, this lesson went much more smoothly this year; the new version of the materials are attached here.

Staircase of Complexity: An Alternative Way to Prove the Exterior Angle Theorem

# Prove exterior angles of triangle

Unit 7: PARALLEL LINES, TRANSVERSALS, AND TRIANGLES
Lesson 7 of 9

## Big Idea: Push kids to prove a harder fact about exterior angles, using transversals.

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Standards:
Subject(s):
Math, Geometry, 8th grade, reasoning and proof, properties of triangles
75 minutes

### Jeff Li MTP

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