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How to Measure Angles with a Protractor

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  • 0:02 Some Background Vocabulary
  • 0:24 The Types of Angles
  • 1:02 Measuring the Angles
  • 1:52 Some Practice
  • 2:24 Lesson Summary
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Lesson Transcript
Instructor: Dina Albert
You're surrounded by angles at home, at school, and even when you're walking down the street. In this lesson, you'll learn how to measure different angles using a protractor.

Some Background Vocabulary

Before we measure angles, we need to be able to recognize them. So, what exactly is an angle? An angle is the figure formed when two lines meet at a vertex (or corner). We measure angles using a protractor, and we use units called degrees, which are the units that measure the value of angles.

The Types of Angles

When you look at the corner of your desk or the corner of a box, you're seeing a right angle, which measures exactly 90 degrees. The straight side of your desk or box measures 180 degrees.

Sometimes angles are smaller than 90 degrees. We call these acute angles. A fun way to remember acute angles is to think, 'What a cute angle!' Here's an example of an acute angle:

acute

Just as there are small angles that are acute, there are bigger angles. Angles that are larger than 90 degrees but not exactly a straight line are called obtuse angles, as we see in a stop sign.

Understanding a Protractor

When we read a protractor, we start at 0, just like we do with a ruler. Protractors have numbers on the outside and on the inside. We read starting from the left if the angle opens to the left hand side, and we read starting from the right if it opens in that direction. It all depends on the angle.

Measuring the Angles

To measure an angle properly, first you can use the straight edge of the protractor to make the lines longer on the angle. This will make measuring easier.

Next, make sure that the black line running along the bottom of the protractor lines up on the bottom of the angle you are measuring.

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