# Lesson 10

Angle Measurement and Perpendicular Lines

### Lesson Purpose

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.

### Lesson Narrative

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.

• Representation
• MLR8

### Learning Goals

Teacher Facing

• Recognize that perpendicular lines meet or cross at a right angle.
• Use a protractor to measure angles.

### Student Facing

• Let’s measure all kinds of angles.

### Required Materials

Materials to Gather

### Required Preparation

Activity 2:

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

### Lesson Timeline

 Warm-up 10 min Activity 1 15 min Activity 2 20 min Lesson Synthesis 10 min Cool-down 5 min

### Teacher Reflection Questions

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?

### Suggested Centers

• Target Measurements (2–5), Stage 4: Degrees (Addressing)
• Compare (1–5), Stage 5: Fractions (Supporting)