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Lesson 2: Angles

# Angles: introduction

Two rays that share the same endpoint form an angle. Learn about angles and the parts of an angle, like the vertex. Created by Sal Khan.

## Want to join the conversation?

• How are Angles used in everyday life?
• Angles are used to design the most basic and the most complex of polygons (shapes). If you look at your clothing, there are many different angles that are used to design skirts and dresses. Look at the collars of your shirts. Diverse angles used in any sort of art design draws the eye. How about stained glass windows?
• how many angles can you think of?
• Besides the 4 types of angles tabron517 mentioned, there are also full and reflex angles. A full angle is a full 360 degrees and a reflex angle is between 180 degrees and 360 degrees. A full angle looks like this: https://upload.wikimedia.org/wikipedia/commons/thumb/a/aa/Full_angle.svg/380px-Full_angle.svg.png An obtuse angle is between 90 and 180 degrees and a straight angle is 180 degrees.

Hopefully you know the rest: acute --> less than 90, right ---> 90. In between, there's an infinite number of possible angles within the possible degrees an angle can have (you can have a 40.34 degree angle for example). If you actually draw angles out you'll find that you can really define an angle to be at either side of a vertex (the point where the two rays meet) and what you would probably consider the "other side" would usually be a reflex angle, which would look something like this: http://www.mathsisfun.com/images/reflexangles.gif
• Is a vertex the meeting point between two rays or two line segments or two lines, or does it not matter?
• Well, if you look at a angle, it has no arrows, but is still is a ray, and still goes on forever, the angle just doesn't have the arrows. I believe that angle are all rays, but I am not sure. It would be good question to google. Hope this was helpful to you!
• At , Sal says you can't call that angle A, then demonstrates another diagram. I know on the 2nd diagram that you can't call it H, but couldn't you call it A in the first one?
• Even Though it is an angle all by itself, the rule still applies.
• how do you tell a 360° angle from a 720° angle or a 45° angle from a 405° angle?
• If you have 720 degrees, you would have two rounds of 360 degrees instead of one. If you 405 degrees, you would have one round 180 degrees and 45 degrees.

Usually, we use degrees under 180 degrees and above 0 degrees.

Hope this helps.
• say that i wanted to name the angle BAC but i wanted name the reflex angle, how would i do it?
• you subtract 360 by the inside angel
• At , Sal says that the point is called AB. Can you call it ray BA?
• Yes! It always stays the same, even if you do switch it from AB to BA.
• why a right angle is ninety degrees?
• Each full circle is 360 degrees. A quarter of that would be a right angle, or 90 degrees.