Angles and Lines

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 1360th of one revolution or turn.

i.e. 360° = one revolution

A minute is defined as 160th 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