The purpose of this lesson is for students to observe that the sum of the interior angle measures of a triangle is 180 degrees. 

The lesson begins with a review of triangle properties followed by an optional activity that reviews congruence. Students identify a set of three congruent triangles, each with a different interior angle measure marked. Students then make observations about the angles in their congruent triangles, including the sum of the three angles.

Next, students decompose a straight angle into three angles and use those angles to create a triangle. Because students see many different combinations of three angles that form triangles, they can use repeated reasoning to make observations (MP8). Students observe a close connection between three positive numbers adding up to 180 and having a triangle with those three numbers as angle measures.

In this lesson, students informally describe the relationship they observe between angle measures in a triangle. They will justify this relationship formally in an upcoming lesson, so they do not need to show that the sum of the angles in a triangle is always 180 degrees at this time.