How to Find the Perimeter of a Circle

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When looking for the perimeter of a circle, remember that the primary term for the perimeter is the circumference. Determine the circumference of a circle by multiplying two times pi and the radius of the circle with help from a math teacher in this free video on basic math lessons.

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Video Transcript

So how does one find the perimeter of a circle? Hi, I'm Jimmy Chang, and I've been teaching college mathematics for nine years. And to find the perimeter of a circle is easy, but you do have to keep in mind that primary use of the word "perimeter" for a circle is actually "circumference." That is actually the term used to describe the distance around a circle, or the perimeter. But we're really trying to find the circumference of a circle. But anyway, here's the formula that you need in order to find circumference. It's going to be C equals two pi R. Now, obviously, two is two. Pi is that number that is approximately 3.14, and R is the radius of a circle. So as long as you know the radius of a circle, then you'll be good to go. So here's a couple of really quick illustrations for you. Suppose you have a circle and you're given that the radius -- the distance from the center to a point on the circle -- is seven. Well, since you already have the radius there, you can just plug it in. So wherever you see the R, just put the seven in. So you'll get C is equal to two pi times R is seven. Since the circumference is a multiplication formula, just multiply across. So two pi times seven...well, two times seven is 14, so it'll be 14 pi for the circumference. That's the exact value. If you want to find the approximate value, just put in 14 pi on your calculator and you'll get more of a decimal answer. Now, the other scenario would be suppose you have another circle and you're given that the distance from one end of the circle through the center to the other is eight. That's the diameter -- that's twice the radius. Now, since you're looking for the radius itself, it's going to be half of eight, which, of course, is going to give you four. So once you have the radius there, you can just plug it in. So C is equal to two pi times...your radius is now four. Again, just multiply across. Two times four is going to give you eight, so you'll get eight pi. So that's a couple of illustrations on how you calculate the perimeter or the circumference of a circle. I'm Jimmy Chang.


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