Lesson 9 Use a Protractor to Measure Angles

- Let’s use some tools to measure angles.

Warm-up True or False: There’s Something about 45

Decide if each statement true or false. Be prepared to explain your reasoning.

$2 \times 45 = 6 \times 15$
$4 \times 45 = 2 \times 90$
$3 \times 45 = 180 - 90$
$6 \times 45 = 45 + 90 + 135$

Activity 1 How Large is a 1° Angle?

Problem 1

A ray that turns all the way around its endpoint and back to its starting place has made a full turn or has turned $360^{\circ}$ .

picture of a ray and circular curve with a label of 3 hundred 60 degrees

What fraction of a full turn is each of the following angle measurements?

$120^{\circ}$
$60^{\circ}$
$45^{\circ}$
$30^{\circ}$
$10^{\circ}$
$1^{\circ}$

Problem 2

Your teacher will give you a protractor, a tool for measuring the number of degrees in an angle.

How is $1^{\circ}$ shown on the protractor?
How many $1^{\circ}$ measurements do you see?

Problem 3

A protractor with no numbers has been placed over an angle.

The center of the protractor is lined up with the vertex of the angle.
The straight edge of the protractor is lined up with a ray of the angle.

How many degrees is this angle? Explain how you know.

angle on protractor without measurement labels. Rays of angle are 12 marks apart.

Problem 4

An angle contains thirty $1^{\circ}$ angles, as shown. How many degrees is this angle?

protractor without number labels measuring an angle. Left side at the forty second marker up from the right. Right side at the twelfth marker.

Activity 2 Use a Protractor

Problem 1

Here are four angles whose sizes you may have estimated earlier. A protractor has been placed over each angle. Measure the size of each angle in degrees.

Problem 2

Elena and Kiran are measuring an angle with a protractor. Elena says the angle is $80^{\circ}$ . Kiran says it shows $100^{\circ}$ . Why might they end up with different measurements? Which one is correct? Explain your reasoning.

protractor measuring an angle. Left side at 0 or 1 hundred 80 degrees. Right side at 100 or 80 degrees.

Practice Problem

Problem 1

Use the given protractor to find the measurement of each angle.