Circles: Circumference & Area

"All You Need is Pi and the Radius"



Listen to this song to learn the formulas for circumference and area of circles. The catchy hook helps students remember that the formula for circumference is "2 pi r" and the formula for area is "pi r squared." The song teaches terms like pi, radius and diameter as it walks students through the process of calculating the circumference and area of example circles.





Flocabulary. Circles. You know how we roll.

You can see me spinnin' on the car,
Circumference = 2πr
I roll with it, boy I’m not scared,
Area = πr2


Squares are square, maybe you’ve found,
I’m not that, you know I get around.
I’m a circle, you know the shape,
But let me tell you what makes me so great.
The distance from my center to my outside,
It’s the craziest, we call that radius
.
All you need in your life and times,
Is the length of my radius and you need π.

Why? With those two you can find,
The circumference of a circle, and that’s no lie.
You can find the area too,
Just follow me, and do what I do.
The circumference is the length all around my outside,
To figure it out just multiply
The radius by 2 and by π.
That’s the circumference, my oh my.

You can see me spinnin' on the car,
Circumference = 2πr
I roll with it, boy I’m not scared,
Area = πr2


Hold up, you want to know how much space
You can find inside of me? Well, OK.
Memorize the formula, so it’s not a barrier,
πr2, that’s my area.
So let’s see, if my radius is 5 ft,
Then what would my area be?
πr2 and r = 5 ft,
So r2 = 25 ft2.

And π is close to 3.14,
So let’s use that as the value for sure.
3.14 · 25 ft2 = 78.5 ft2.
Now, all you need in your life and times,
Is the length of my radius and you need π.
My radius, that’s just 12 my diameter,
Like a boss, 'cause you’re talking to the manager.

You can see me spinnin' on the car,
Circumference = 2πr
I roll with it, boy I’m not scared,
Area = πr2


What is the radius ? Radius is the distance from the center of the circle to the outer edge. This is not to be confused with the diameter , which is the length across a circle. The radius is always one half the diameter.


What is the circumference ? The circumference is the perimeter of a circle, or the distance around the outside of it.

To find the circumference of a circle, use the formula 2πr. This can also be written as πD, since D = 2r.

What is pi? Pi is the relationship between a circle's diameter and its circumference. This relationship is always the same, and it's always Pi. Pi is an irrational number, which means its decimals go on forever, but it is approximately 3.14. Want to get a little more specific? Talk to Lu Chau, the student in China who was able to memorize the first 67,000 digits of pi, in order, without making a mistake.


‪If you have to solve a problem that involves a radius but are only given the diameter, just remember that you can still use the diameter to solve the problem, since the radius is equal to half of the diameter. The same is true if you need the diameter but are only given the radius.


To find the area of a circle, use the formula πr 2 . It's important to remember to square the radius because area is always measured in square units.

What is the radius of a circle?
The distance from its center to its outer edge
What is the circumference of a circle?
The perimeter or length around the outside of it
What is the formula for the circumference of a circle?
C = 2πr
What is the formula for the area of a circle?
πr 2
What is the approximate value of pi?
3.14

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