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.
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
. 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.
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.