1.5. Angle Measurement http://www.ck12.org
1.5 Angle Measurement
Here you’ll learn how to measure an angle with a protractor and how to apply the Angle Addition Postulate to find
unknown values.
What if you needed a way to describe the size of an angle? After completing this Concept, you’ll be able to use a
protractor to measure an angle in degrees.
Watch This
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Click image to the left for more content.CK-12 Foundation: Chapter1MeasuringAnglesA
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Click image to the left for more content.James Sousa:Animation of Measuring Angles with a Protractor
Watch the first part of this video.
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Click image to the left for more content.James Sousa:Angle Basics
Guidance
We measure a line segment’s lengthprotractor. A protractor is a measuring device that measures how “open” an
angle is. Angles are measured in degrees, and labeled with a◦symbol.
Notice that there are two sets of measurements, one opening clockwise and one opening counter-clockwise, from 0◦
to 180◦. When measuring angles, always line up one side with 0◦, and see where the other side hits the protractor.
The vertex lines up in the middle of the bottom line, where all the degree lines meet.