An angle measures the amount of turn
Names of Angles
As the Angle Increases, the Name Changes:
|Type of Angle||Description|
|Acute Angle||an angle that is less than 90°|
|Right Angle||an angle that is 90° exactly|
|Obtuse Angle||an angle that is greater than 90° but
less than 180°
|Straight Angle||an angle that is 180° exactly|
|Reflex Angle||an angle that is greater than 180°|
Try It Yourself!
In One Diagram
This diagram might make it easier to remember:
Also: Acute, Obtuse and Reflex are in alphabetical order.
Be Careful What You Measure
This is an Obtuse Angle
And this is a Reflex Angle
But the lines are the same ... so when naming the angles make sure
Positive and Negative Angles
When measuring from a line:
- a positive angle goes counterclockwise (opposite direction that clocks go)
- a negative angle goes clockwise
Parts of an Angle
The corner point of an angle is called the vertex
And the two straight sides are called arms
The angle is the amount of turn between each arm.
How to Label Angles
There are two main ways to label angles:
1. give the angle a name, usually a lower-case letter like a or b, or sometimes a Greek letter like α (alpha) or θ (theta)
2. or by the three letters on the shape that define the angle, with the middle letter being where the angle actually is (its vertex).
Example angle "a" is "BAC", and angle "θ" is "BCD"