An Introduction to Myself - Math Activity
Part IV

How do we measure circles? Hmmmm...

In a way, a circle is like a square. Why? Because it is as tall as it is wide. If you draw a vertical line ( | ) across the middle of a circle, it will measure the same as a horizontal line ( -- )going through the middle of the same circle.

What do we call the line that goes from side to side through the middle of a circle? The answer is.... DIAMETER ( The stress is on the "A")!

If a circular shape is longer in any direction, it is no longer a circle. 

Open a new play-slide in your Power Point presentation. We'll delete it later.

Select the BLANK slide layout.

On your drawing toolbar, select the circle. NOTE: To draw a perfect circle, hold down the SHIFT key as you drag. 

How can you check to see if you drew a perfect circle? Fill the information in the sentence below.


 (Click on the arrow to review ratios and proportions in our last task. )

With the FORMAT OBJECT panel open on the Size Tab, make the circle half as small.

The line around the circle is called the CIRCUMFERENCE--long word for a big job!

How do we measure the circumference? Hmmmm. It's easy to measure the outside of a square or rectangle. We just add the dimensions of each side. But a circle doesn't have different sides to add.

In order to measure the dimensions of a circle, you need to learn two more terms: radius and PI.

The radius of a circle in the measurement from the middle of the circle to any of its sides. In other words, the radius is half (1/2) of the diameter

And what is PI? PI represents the steady measurement of the distance around a circle divided by the distance across its center. Don't worry much about that. Just remember that when you want to find out how much the outside of your circle (circumference) measure, you need to use the PI value. That value is approximately 3.14. That's it. We use the Greek letter [IMAGE] (Pi)  to represent this 3.14 value.

Here are some formulas for you to play with.

Let's call CIRCUMFERENCE "C" so we don't have to write the big word every time.

Let's call RADIUS "R."

Let's call PI, [IMAGE].

And let's call DIAMETER "D."

Here we go.

C = [IMAGE] ˇ D (The dot in the middle is the same as times or X)

D = 2 ˇ R

R = D ÷ 2 ( or D/2)

If a tire has a diameter of 3 feet, what is it's radiusR = D ÷ 2 ( or D/2)

Three divided by 2 equals 1.5 feet!

It's as simple as that!

To practice these formulas, click on the following sites and do their exercises. They are a lot of fun. 


Print this page, sign it,  and place it in your portfolio. 

When you are finished, delete the slide with the circles you dress and save your presentation again.

Funded by the Colorado Department of Education through the Cortez Adult Education Program. All rights reserved.
ŠUnlimited Learning, 2004

Leecy Wise, author