# Prove exterior angles of triangle

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## Objective

SWBAT to prove a fact about exterior angles of a triangle.

#### Big Idea

Push kids to prove a harder fact about exterior angles, using transversals.

10 minutes

40 minutes

## Huddle

10 minutes

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.

## Homework

15 minutes

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