How_To_Be_Good_At_Math

GEOMETRY • CIRCLES 221

Answer on page 320

Chord

A line between two points

on the circle’s circumference that

doesn’t go through the centre.

Segment

The space between a

chord and an arc.

Tangent

A straight line that touches

the circumference at one point.

Area

The amount of space inside

the circle’s circumference.

Circumference

The distance all the way

round the circle. It’s the circle’s

perimeter.

Radius

A straight line from the

centre of the circle to the

circumference. The plural of

“radius” is “radii”.

Arc

Any part of the circle’s

circumference is called an arc.

Diameter

A straight line from one side

of the circle to the other, going

through the centre. The diameter

is twice the length of the radius.

Sector

A slice of the circle formed

by two radii and an arc.

The diameter

divides a circle

in half

TRY IT OUT

Measure the circumference

A ruler won’t help us measure a circle’s

circumference – it can’t measure curves!

But luckily, we can find the circumference

of any circle if we multiply its diameter

by 3.14.

First, measure the diameter

of this wheel, then multiply

the diameter by 3.14 to work

out the circumference.

Now put some string round

the circumference, then

measure the string with a ruler.

Do you get the same answer?

Chord

Use a ruler to

measure the diameter

220_221_circles.indd 221 29/02/2016 18:04