Lesson 15

The purpose of this lesson is to use known angles to reason about unknown angles and write equations to reason about unknown angles.

In this lesson, students use what they know about angles and their measurements to solve problems that are increasingly more complex and abstract. To find measurements of unknown angles, students need to look for structure in the diagrams, reason about the relationships of the angles (including writing equations to represent the relationships), and perform addition, subtraction, and sometimes division.

The problems in the lesson can be solved in more than one way and in different orders, but a small handful of the angles can only be quantified after the values of some other angles are known. Students pay attention to the process and explain why sometimes a certain sequence is necessary.

• Create a set of cards from the blackline master for each group of 2.

As you finish up this unit, reflect on the norms and activities that have supported students’ development as thinkers. How have students grown in their ability to reason more abstractly and logically? What instructional strategies or routines worked well in supporting this development? What will you continue to do and improve on in Unit 7, and what will you adjust?