Lesson 10

The purpose of this lesson is for students to practice using a protractor to measure angles and to learn that lines that intersect at a right angle are perpendicular lines.

In a previous lesson, students learned that a 1-degree angle measures a turn through ​\(\frac{1}{360}\)​ of a circle and that a protractor could be used to measure angles in degrees.

In this lesson, students practice using a protractor to measure a variety of angles—angles formed by rays or line segments and those that are in other two-dimensional figures. Students consider how to position the tool, which set of numbers to use, and whether their measurements make sense.

Next, students are prompted to fold paper to create two lines that form right angles. They learn that intersecting lines that form \(90^\circ\) angles are perpendicular. They then practice identifying perpendicular lines and segments.

• Prepare at least 2 pieces of paper (or sticky notes) for each student.

In the past few lessons, students may have made assumptions (correct or incorrect) about angle sizes based on their appearance. In this lesson, they may have done the same when looking for perpendicularity. What questions can you ask, or what instructional moves can you consider, to encourage students to check their assumptions?