Page 55 - Mathematics Coursebook

5.1 Labelling and estimating angles

5.1 Labelling and estimating angles

Some important angle facts

A half turn is 180°. 180°

A quarter turn is 90° and is called a right angle.
90°

An acute angle is less than 90°.

An obtuse angle is between 90° and 180°.

$is diagram shows parts of lines between two points, AB, AC and AD. B
You can label a line segment by writing down the letters of the points at
each end of the line. A

You can write the points in any order: AB and BA are two di!erent C
ways to label the same line segment. D
If you look again at the diagram, you can see several angles.

$e angle between AB and AC is called angle BAC or angle CAB. $e letter of the point of the angle is

always in the middle. B
Here is part of the diagram again.
$ere are two angles at A between AB and AC. A

One is an acute angle. $e other angle is more than two right angles. C
An angle that is more than two right angles is called a refl ex angle.
Normally, if you refer to ‘angle BAC’, you would mean the smaller Note that the arc for a refl ex
of the two angles. If you want to refer to the other one, you must angle goes around the outside.

call it ‘re"ex angle BAC’.
Worked example 5.1

Angle CDE is a right angle. How big is refl ex angle CDE?

360° – 90° = 270°
D C
Refl ex angle CDE is 270°.
The two angles at D add up to 360°; 360° – 90° = 270°. 90°

E

54 5 Angles