Circumference of circle diameter 7 cm

Step by step

The question

Question: find the circumference of a circle with diameter 7 cm.

1 Find the radius

The diameter goes all the way across, through the middle.

The radius goes from the middle to the edge.

It is half of the diameter.

Half of 7 is 7 divided by 2.

7 divided by 2 is 3.5.

So r = 3.5 cm.

2 Use the rule

π is a special number for circles.

It is about 3.14.

The circumference is the way around the circle.

It is 2 times π times the radius: C = 2 × π × r.

So we write C = 2 × π × r.

Now put the numbers in: r = 3.5.

2 times 3.5 is 7.

We write it as 7π.

We keep π in the answer.

That is the exact answer.

3 About how much

To see about how much, put 3.14 for π.

7 times 3.14 is 21.98.

So the circumference is about 21.98 cm.

Let's look back.

First: find the radius.

Then: use the rule.

Last: about how much.