Lesson 8

The Size of Angles in Degrees

Warm-up: What Do You Know about 360? (10 minutes)

Narrative

The purpose of this warm-up is to invite students to think about 360 in terms of related numbers—as a result of addition, subtraction, multiplication, or division. The reasoning done here will be helpful when students compose angles into a sum of $$360^\circ$$ or decompose a $$360^\circ$$ angle into smaller angles, particularly benchmark angles such as $$30^\circ$$, $$60^\circ$$, $$90^\circ$$, and $$180^\circ$$.

Launch

• Display the number.
• “What do you know about 360?”

Activity

• 1 minute: quiet think time
• Record responses.
• If no students mentioned different ways to express 360, ask: “How could we express the number 360?” and “What do you know about the factors of 360?”

Student Facing

What do you know about 360?

Student Response

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Activity Synthesis

• Draw students’ attention to the factors of 360. “What are the factors of 360? How many are there?”
• “The number 360 and its factors are important when describing angles. Let’s find out why they show up again and again as we look at a new way to describe and measure the size of angles.”

Activity 1: A Full Turn (15 minutes)

Narrative

In previous activities, students used the features of a clock to describe and compare angles. This activity introduces students to degree as a unit of measure.

Because one degree is much more abstract than one inch or one square inch tile, students are first introduced to 360 degrees as a full rotation of a ray around its endpoint. Students use this information to reason about other angle measurements. They may use their understanding of fractions of a circle to determine the sizes (MP7).

In the synthesis, students describe angle measurement as additive. Students will continue to build this understanding and reason about the size of 1 degree in subsequent lesson activities.

Launch

• Groups of 2
• “Just like length and area can be measured in standard units, angles have standard units.”
• “One standard unit for measuring the size of angles is degree.”
• Display the image of the ray turning 360 degrees and read the opening task statement.

Activity

• “Use this information to determine how many degrees each ray has turned from where it started. Then sketch some angles that are about the size given in degrees.”
• 3–4 minutes: independent work time
• “Compare your thinking with your partner. Explain how you made your estimates.”
• 2–3 minutes: partner discussion
• Monitor for students who:
• describe the 180 degree turn as half a full turn or turning half a circle
• describe the 90 degree angle as half of half a turn or as a turning a fourth of the way around the circle
• use their estimate for the 90 degree angle to estimate the 270 degree angle by adding ($$90 + 90 + 90$$ or $$180 + 90$$) or by subtracting ($$360 - 90$$)

Student Facing

A ray that turns all the way around its endpoint and back to its starting place has made a full turn.

We say that the ray has turned 360 degrees.

1. How many degrees has the ray turned from where it started?

2. Sketch two angles:

1. an angle where a ray has turned $$50^\circ$$
2. an angle where a ray has turned $$130^\circ$$

Student Response

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Activity Synthesis

• Invite previously identified students to share how they estimated the turn of each ray in degrees.
• “How were your methods the same? How were they different?” (We all used the first diagram to figure out how much of a turn. Some of us used one measure to find the next measure. Some of us thought about addition and some thought about division).
• Consider displaying the equation $$90 + 90 + 90 = 270$$ and discussing:
• “How does this match a way to estimate the third angle?”
• “What’s another equation we could use to describe that angle?”
• “An angle that measures 90 degrees is called a right angle.”
• “Where have you seen right angles before?” (Corners of squares or rectangles. Corners of paper. Angle made by the hands of a clock when it is 3 o'clock or 9 o'clock.)

Activity 2: Make a Measuring Tool (20 minutes)

Narrative

In this activity, students construct a protractor-like tool that shows some benchmark angles. They do so by halving given angles—$$120^\circ$$ and $$180^\circ$$—and then of subsequent angles identified along the way.

The activity serves several goals. The first is to familiarize students with the structure of a protractor using tactile processes (folding paper and aligning lines or edges). The second goal is to develop students’ intuition for thinking of a larger angle as composed of smaller angles, preparing them to see (in future lessons) that a $$1^\circ$$ angle is $$\frac{1}{360}$$ of a full turn. A final goal is to motivate the need for a tool that can measure angles more precisely.

Some students may use a square corner of a sheet of paper to find a $$90^\circ$$ angle on their semi-circle and others may choose to fold their semi-circle in half. Expect most students to fold their paper to find all subsequent angles.

Required Materials

Materials to Gather

Materials to Copy

• Making a Measuring Tool

Required Preparation

• Create a paper half-circle from the blackline master for each student.

Launch

• Groups of 2–4
• Give each student one paper half-circle and access to rulers or straightedges.
• “Your sheet of paper is in the shape of half a circle. It shows a ray on the bottom right and two angles ($$120^\circ$$ and $$180^\circ$$) measured from the ray.“
• “We see the $$120^\circ$$ label. Where is the $$120^\circ$$ angle? Where are the two rays that make this angle?”
• 1 minute: quiet think time for the first problem
• 1 minute: group discussion
• “Where do you think the second ray of a $$90^\circ$$ angle would be?” (Between 0 and 120, but closer to 120.)

Activity

• 5–7 minutes: independent work time
• As students work on the last problem, monitor their ideas for using their tool to estimate angle measurements.

Student Facing

Your teacher will give you a sheet of paper in the shape of half a circle. It shows a $$120 ^\circ$$ angle and a $$180^\circ$$ angle from the ray on the bottom right.

On the half-a-circle paper:

1. Draw a line segment to show a $$90^\circ$$ angle from the same ray. Label it with the measurement. Be as precise as possible.
2. Draw lines to show the following angles (measured from the same ray). Label each line with the measurement.

• $$60^\circ$$
• $$45^\circ$$
• $$30^\circ$$
• $$150^\circ$$
• $$135^\circ$$
3. Can you find a $$1^\circ$$ angle from that same ray? Explain or show how you might do it.
4. You just made a measuring tool!

How can it be used to estimate the size of an angle? Discuss your ideas with your group. Then, use your tool to estimate the sizes of at least two angles.

Student Response

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Advancing Student Thinking

Students may create angles that are not precise when they estimate where to draw a new line segment. Ask the students to explain how they determine where to draw a line segment and suggest folding as a strategy. Consider asking:

• “Do you think the angle you need to create is smaller or larger than those that you have already drawn? How much smaller or larger?”
• “How might folding help you create more precise angles during this task?”

Activity Synthesis

• “How did you find a $$90^\circ$$ angle?” (I folded the semi-circle into halves.)
• “How did you find all the other angles?” (For $$60^\circ$$, fold to line up the thick ray with the $$120^\circ$$ line, splitting 120 into two. For $$45^\circ$$, fold to line up the ray with the $$90^\circ$$ line, splitting 90 into two. Repeat in a similar fashion to find the others.)
• Invite students to share how they might find a $$1^\circ$$ angle on their half circle. Highlight explanations that involve finding some fraction of increasingly smaller and smaller angles.
• Solicit some estimates of the angle measurements in the last problem. Record and display them for comparison later, when the same four angles are measured using a protractor.
• Students are likely to notice that their tool is imprecise and is not reliable or practical for measuring angles beyond estimations. Explain that we will look at another tool in the next activity.

Lesson Synthesis

Lesson Synthesis

“In earlier lessons, we used clocks to help us compare angles and talk about their size. Today, we learned that angles can be measured in degrees and that some angles are helpful for estimating the size of other angles.”

“What did you learn about $$360^\circ$$?” (It’s the measurement of the angle made by a ray that makes a full turn around a point.)

“How would you describe a $$180^\circ$$ angle?” (It’s half of a full turn. The two rays make a straight line or point in opposite directions.)

“How would you describe a $$90^\circ$$ angle?” (It’s a half of a half turn. It’s the size of an angle made when a ray makes a fourth of a full turn around a point. It is called a right angle. It’s the size of angle at the corners of a piece of paper.)

Cool-down: Estimate Angle Size in Degrees (5 minutes)

Cool-Down

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