Idiot\'s Guides Basic Math and Pre-Algebra

Chapter 12: Basics of Geometry 155

A number line like this can be used to measure line segments. For example, the length of line segment

AB is equal to the distance between coordinates -7 and -2, or five units.

You can also measure angles. Angles are measured by the amount of rotation from one side to
the other. Picture the hands of a clock rotating, creating angles of different sizes. It is important
to remember that the lengths of the sides have no effect on the measurement of the angle. The
hands of the famous clock known as Big Ben are much longer than the hands of your wrist watch
or alarm clock, but they all make the same angle at 9 o’clock.

So how do you put a ruler on an angle? For starters, it’s not a ruler. A ruler is a line you use to
measure parts of lines. Angles aren’t parts of lines. They’re more like wedges from a circle. So to
measure them you create an instrument called a protractor, a circle broken into 360 little sections,
each called a degree.

In geometry, angles are measured in degrees. When you put the protractor over the angle with
the center of the circle on the vertex of the angle, the sides fall on numbers, called coordinates.
The measure of the angle is the absolute value of the difference of the coordinates.

DEFINITION

A protractor is a circle whose circumference is divided into 360 units, called degrees,

which is used to measure angles.

A protractor can be created using any circle, but most people are familiar with the plastic half-circle

tool shown here.

-10 -9 -8

A

-7 -6 -5 -4 -3 -2 -1 0

x

12345678910

B C D E F

Z

W

X Y