Points, lines and angles  (Page 2/3)

Using a protractor

A protractor is a simple tool that is used to measure angles. A picture of a protractor is shown in [link] .

Method:

Using a protractor

1. Place the bottom line of the protractor along one line of the angle so that the other line of the angle points at the degree markings.
2. Move the protractor along the line so that the centre point on the protractor is at the vertex of the two lines that make up the angle.
3. Follow the second line until it meets the marking on the protractor and read off the angle. Make sure you start measuring at 0 ${}^{\circ }$ .

Measuring angles : use a protractor to measure the following angles:

Special angles

What is the smallest angle that can be drawn? The figure below shows two lines ( $CA$ and $AB$ ) making an angle at a common vertex $A$ . If line $CA$ is rotated around the common vertex $A$ , down towards line $AB$ , then the smallest angle that can be drawn occurs when the two lines are pointing in the same direction. This gives an angle of 0 ${}^{\circ }$ . This is shown in [link]

If line $CA$ is now swung upwards, any other angle can be obtained. If line $CA$ and line $AB$ point in opposite directions (the third case in [link] ) then this forms an angle of 180 ${}^{\circ }$ .

An angle of 90 ${}^{\circ }$ is called a right angle . A right angle is half the size of the angle made by a straight line (180 ${}^{\circ }$ ). We say $CA$ is perpendicular to $AB$ or $CA\perp AB$ . An angle twice the size of a straight line is 360 ${}^{\circ }$ . An angle measuring 360 ${}^{\circ }$ looks identical to an angle of 0 ${}^{\circ }$ , except for the labelling. We call this a revolution .

Angles larger than 360 ${}^{\circ }$

All angles larger than 360 ${}^{\circ }$ also look like we have seen them before. If you are given an angle that is larger than 360 ${}^{\circ }$ , continue subtracting 360 ${}^{\circ }$ from the angle, until you get an answer that is between 0 ${}^{\circ }$ and 360 ${}^{\circ }$ . Angles that measure more than 360 ${}^{\circ }$ are largely for mathematical convenience.

Once angles can be measured, they can then be compared. For example, all right angles are 90 ${}^{\circ }$ , therefore all right angles are equal and an obtuse angle will always be larger than an acute angle.

The following video summarizes what you have learnt so far about angles.

Special angle pairs

In [link] , straight lines $AB$ and $CD$ intersect at point X, forming four angles: $\widehat{X_1}$ or $\angle BXD$ , $\widehat{X_2}$ or $\angle BXC$ , $\widehat{X_3}$ or $\angle CXA$ and $\widehat{X_4}$ or $\angle AXD$ .