# Triangle Sum Theorem and Special Triangles

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## Objective

Students will be able to discover and prove the triangle sum theorem.

#### Big Idea

By investigating the angles of four different types of triangles, students conjecture about the interior angle sum of triangles and ultimately prove the triangle sum theorem.

## Warm-Up: Parallelogram Sides and Proof

20 minutes

In this Warm-Up, students write and solve algebraic equations to review the idea that the opposite sides of a parallelogram are congruent.  While I circulate the room, I look for a student who will volunteer to present their work on the first problem.  At this point, I want a student who can give a model explanation for how they used the given information from the problem (the quadrilateral is a parallelogram) to draw certain conclusions (the opposite sides are congruent, so I can write and solve equations).

When they construct a proof, I want my students to begin by using the given information to draw their own diagram.  As I circulate the room, I want to find a student who can logically show why the vertical angles are congruent by using the ideas we discussed in our work with the Language and Properties of Proof during yesterday's last lesson.

## Triangle Sum Investigation and Practice

25 minutes

During the Triangle Sum Investigation, students work in groups of four to "discover" the interior angle sum of a triangle.  The structure of this investigation requires each student to take on a different “case” to explore, compare results, and then draw conclusions.  In this investigation, the group collectively explores the angles of acute, right, obtuse, and isosceles triangles to then conjecture about the interior angle sum of any triangle.

I include a check-in point for this investigation, which provides me with an opportunity to assess student understanding.  Groups call me over to check in, which essentially involves one student explaining what the group has conjectured.  I like to shuffle all of my students’ papers behind my back to “randomly” choose the student who I will question—since students are familiar with this structure, they prepare each other for this moment, which holds them accountable to the group’s learning.  After the check-in, I ask groups to move on to the next part of the task, which gives them an opportunity to apply and practice what they have just discovered.

## Debrief Notes

15 minutes

As in my other lessons, I like to formally debrief the discoveries we have made by having students take notes in their note-takers.  We discuss students’ conjecture about the interior angle sum of any triangle, and extend this understanding to consider other conclusions we can draw about the angles of triangles.  I introduce the idea of an auxiliary line to students, which they need when proving the Triangle Sum Theorem.

## Prove the Triangle Sum Theorem

20 minutes

In the Triangle Sum Theorem Proof, I ask students to construct a parallel line to the base of the triangle. I choose to do this because students, through construction, have to consider the angle relationships that will yield parallel lines, which gives them a way into the proof.  (For example, students might use alternate interior angles to construct their auxiliary line, which then gives them a possible path to pursue.)

Because the proof of the triangle sum theorem requires students to go out of their comfort zone (work with auxiliary lines, name extra angles that they might use), I have found it really important to have at least two different students present their work (MP1).

• Many students use alternate interior angles in this proof and make connections to the Triangle Sum Investigation, where students arranged the angles of a triangle on a straight angle.
• An interesting proof that students often arrive at involves using consecutive interior angles to show that the interior angles of a triangle are supplementary.

## Exit Ticket: Explain to a 4th Grader

5 minutes

In this Exit Ticket, I want to assess students' understanding of the Triangle Sum Theorem.  I want them to "explain to a 4th grader" because I want to get a good sense of how clearly they can articulate their understanding of the triangle sum theorem in a way that would make sense to anyone.