PolygonsinBrisbane.jpg

Notation

An angle is formed when two lines or rays meet at the same point. The meeting point is called thevertex of the angle.

The angle formed is written as Y7_Angles_01.gifBAC or BÂC.

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. Â or aY7_Angles_04.gif


 

Measurement of Angles

A degree is defined as Y7_Angles_05.gifof one revolution or turn.

i.e. 360° = one revolution

Angles can be measured using a protractorY7_Angles_06.jpg (see Topic 29).


 

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.Y7_Angles_07.gif

Angles that lie within a certain range of values are given special names.

 

Angle size
Name

Examples

Between 0° and 90°

Acute
Y7_Angles_09.gif

90°

Right

 

Y7_Angles_10.gif

Between 90° and 180°

Obtuse

 

Y7_Angles_11.gif

180o

Straight

 

Y7_Angles_12.gif

Between 180° and 360°

Reflex

 

Y7_Angles_13.gif

 

Properties of Angles

Adjacent angles on a straight line

 

Property

Diagram

Adjacent angles on a straight line add up to 180°.

 ( Adj. Y7_Angles_01.gifs on st. line add up to 180°)

 
Y7_Angles_14.gifa + b = 180°

Example

Y7_Angles_15.gif

Find the value of a

a + 69 + 41 = 180
( Adj. Y7_Angles_01.gifs on st. line add up to 180°)

a + 110 = 180
a = 180 − 110
a = 70o

Angles around a point

Property

Diagram

Angles around a point add up to 360°

(Y7_Angles_01.gifs at a pt. add to 360°
Y7_Angles_16.gif      c + d + e = 360°

  Example

Y7_Angles_17.gif

z + 65 + 154 = 360
(Y7_Angles_01.gifs at a pt. add to 360°

z + 219 = 360
z = 360 − 219
z = 141o

Vertically opposite angles

Property

Diagram

Vertically oppostite angles are equal or congruent.

(Vert. opp. Y7_Angles_01.gifs equal)

Y7_Angles_18.gif

f = g and h = i

Example

Y7_Angles_19.gif

y = 119o (Vert. opp. Y7_Angles_01.gifs equal)