Prove exterior angles of triangles 2

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SWBAT prove facts about the exterior angles of a triangle.

Big Idea

More practice proving a tough-to-prove fact about triangles, using transversals.

Warm Up

10 minutes

Each day, students complete a warm-up that usually consists of spiraling the previous day's material, in addition to older material. Warm-up problems also sometimes extend lessons that students have encountered before to more unfamiliar contexts.

For a video narrative about how I structure each lesson, and how the warm-up fits in, click here.

Play of the Day

40 minutes


10 minutes

Today is about getting more and more kids fluent in their ability to prove this fact, and so I ask a few kids to come up to the board to present proofs, 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?

At this point, students are developing the habits of mind needed to be strong proof-writers.  They establish certain facts, choose the logical and correct sequencing of each step, and make a strong conclusion based on their reasoning.  We talk about these three elements as the three elements that form the basis of a strong proof.


15 minutes

Homework 98 is to be completed at home and should take students approximately 15 minutes.