ShmoopTube

00:03

Here’s a question that's part of a balanced Shmoopy breakfast.

00:06

A circle has an area of 100π cm2. What is the circumference of the circle?

00:13

And here are the potential answers...

00:16

All right…seems to us we’re going to have to know a formula or two to solve this puppy.

00:24

We should know that the area of a circle is πr2…

00:26

…and that the circumference of a circle is 2πr.

00:31

The problem has graciously told us the area, so we should be able to figure out what r is,

00:35

and then just plug ‘er into the circumference formula.

00:38

Okay…so by swapping out what we’re given with what we know…

00:41

…we get that 100π equals πr2.

00:44

We can divide pi out of both sides to give us 100 equals r squared…and then take the

00:48

square root of both sides to get r = 10.

00:51

All right, we can take our new r value and move it over to the circumference formula…

00:56

…which becomes 2 pi times 10…or 20 pi.

00:59

Which is answer D.

01:00

Wow…20 pi… that’s not just a pi… that’s a whole bakery.

Full Transcript