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Lesson 9: Angles

# Measuring angles in degrees

Learn how to measure an angle with a protractor.   Created by Sal Khan.

## Want to join the conversation?

• can there be a 190 degree angle?
• There can be more than 190 degrees, in fact there can be more than 360 because the sum of all inter-angles of a hexagon is 540. My suggestion is to watch snowboarding on the winter X-games and hear the commentators carefully for numbers. Those numbers are degrees.
• what hapens if an angle is more than 180 degrees? what kind of angle is that?
• An angle greater than 180 degrees is called a reflex angle. It doesn't come up in conversation too often, but maybe you can win a trivia contest someday if you remember that. :)
• Is there a 200 degrees angle
• Yes, because angles can go up to 360 degrees. You know on those circle protractors that help you measure an angle? They have degrees going all the way up to 360. 360 degrees would be a full circle.
• at , Sal says "0-degree angle," wouldn't it be a 180-degree angle?
• Imagine a book placed in front of you with the back of it to your left. When the book is closed, it is open at a 0 degree angle. When you open it halfway, it is open at a 90 degree angle. When you open it all the way, it is open at a 180 degree angle. when you break the spine of the book by opening it so far that the front and back cover are touching each other, it is open at a 360 degree angle.
• Since a right angle is considered 90°, isn't it safe to say that a 270° angle should be called a left angle?
• An angle between 180 degrees and 360 degrees is called an reflex angle
(1 vote)
• a unit of angle, equal to an angle at the center of a circle whose arc is equal in length to the radius.
• Is there a reason that 90 degree angles are called right angles?
• They are half of 180 and that they are perpendicular to the other line.
Another reason is simple.
It was named like that, so there is no reason and that would make it a hard question.

Very good question
• what if there was a 361 angle?
• A 361° angle is a perfectly valid concept. The thing is that a 361° angle is in a lot of ways the same thing as a 1° angle. This is a case in math where the meaning depends a lot on the context: if freestyle skier goes off a jump and spins around 361°, that means that they did slightly more than one full rotation; this is not the same thing as the skier going off the jump and spinning 1°, which would just be a tiny bit of rotation. In abstract math like trigonometric analysis, an angle of 361° often equals an angle of 1°, but you have to be careful about how you interpret the situation.
• can you have a 360 drgree in a triangle