Calculating the Perimeter of a Circle

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Calculating the perimeter of a circle, which is known as the circumference, can be done by multiplying the number pi by two times the radius. Find the circumference of a circle given either the radius or diameter with instructions from a college-level math teacher in this free video on geometry.

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Video Transcript

So, how do you calculate the perimeter of a circle? Hi, I'm Jimmy Chang. I've been teaching college mathematics for nine years, and when we think about the perimeter of a circle, it's actually not used as frequently because a perimeter of a circle actually has another name for it and that is actually the circumference of a circle, that is the more widely commonly used notation. So when we think about the perimeter of a circle, the distance around the circle, we are really talking about circumference of a circle. So that's the formula we're going to be using today. Now, the circumference of a circle is fairly straight forward. It's 2 times pi, which is that famous number, 3.14 approximately, times r, which is the radius of a circle. So here's a little case in point. Suppose you have a circle of radius 7 and you want to find circumference, all you need to do is put the 7 in place of the r, so you will have c is equal to 2 times pi, times 7. Now, all you have to do here is because it's all multiplication, just multiply the whole numbers first, 2 times 7, and you'll have 14 pi as the circumference, that means the distance around exactly is 14 pi, but if you want to find a decimal answer just go ahead and enter 3.14 where the pi is and you can have a more approximate answer as to the circumference of a circle. Now, the only other thing that they might throw at you when finding the circumference of a circle is, if they give you another circle, and let's just say the distance from one end of the circle to the other is 10. Well in that case you still need to find the radius, but that's actually pretty straight forward to do because if the diameter, which is what the distance across is 10, then the radius has to half that, which is 5. And to find the circumference there, you just plug in the 5 where the r is, and you have 2 times pi times 5, again multiply the whole numbers together, 2 times 5 is 10 and you have the exact value of the circumference being 10 pi, or if you want to find the decimal answer, just put 3.14 in place of pi to get and idea as to what the approximation is. So, I'm Jimmy Chang and that's exactly how you find the perimeter or the circumference of a circle.

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