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Everything Geometry

Introduction to Angles

Students should be able to:

  • identify and describe the relative size of acute, obtuse and straight angles with respect to right angles.

What is an angle?

When two rays share an endpoint they form an angle. The point where the rays intersect is called the vertex of the angle. The two rays are called the sides of the angle.

rays and vertex

Get Adobe Flash player

Click on the SEE THE VERTEX AND RAYS button above to see the different parts of an angle.

How do you measure an angle?

We measure the size of an angle by looking at how much the two sides move, or rotate, away from each other. As you can see below, the sides rotate on the vertex.


The unit of measure we use to show how much the sides have rotated is the degree. In the mini-movie below you can see that the angle rotates to a stop at four different measures:
  • 45 degrees - An angle that rotates less than 90 degrees. Also called an acute angle.
  • 90 degrees - An angle formed when the rays are perpendicular. Also called a right angle.
  • 135 degrees - An angle that rotates more than 90 degrees. Also called an obtuse angle.
  • 180 degrees - Also called a straight angle.
rays and vertex

Get Adobe Flash player

Click on the PLAY button above to see the angle stop at four different measures. Click the STOP button to pause it at any time.



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