Notation
Two lines intersect at A. The intersection of these two lines forms an angle.
The angle formed is written as BAC or BC.
The vertex of the angle is always represented by the middle letter.
A single letter can also be used to represent an angle e.g. or a
Measurement of Angles
A degree is defined as 1⁄360th of one revolution or turn.
i.e. 360° = one revolution
A minute is defined as 1⁄60th of a degree.
i.e. 1 degree = 60 minutes (1° = 60')
Angles can be measured using a protractor.
Types of Angles
- Complementary angles add up to 90o.
- Supplementary angles add up to 180o.
- Congruent angles are equal.
- Adjacent angles are next to one another and have a common vertex and line.
p and q are adjacent angles.
Angles that lie within a certain range of values are given special names.
Angle size |
Name |
Examples
|
Between 0° and 90°
|
Acute |
|
90°
|
Right
|
|
Between 90° and 180°
|
Obtuse
|
|
180o
|
Straight
|
|
Between 180° and 360°
|
Reflex
|
|
Properties of Angles
Property
|
Diagram
|
Adjacent angles on a straight line add up to 180°
|
a + b = 180° |
Angles around a point add up to 360°
|
c + d + e = 360° |
Vertically opposite angles are equal.
|
f = g, h = i |
|