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The circumference of a circle can be found by multiplying pi ( π = 3.14) by the diameter of the circle.

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The area of a circle can be found by multiplying pi ( π = 3.14) by the square of the radius.

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Area

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Circumference

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Sal uses formulas and a specific example to see how area and circumference relate.

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- [Teacher] A candy machine creates small chocolate wafers in the shape of circular discs. The diameter, the diameter of each wafer is 16 millimeters. What is the area of each candy? So, the candy, they say it's the shape of circular discs. And they tell us that the diameter of each wafer is 16 millimeters. So if I draw a line across the circle that goes through the center, the length of that line all the way across the circle through the center is 16 millimeters. So let me write that, so diameter, the diameter here is 16 millimeters and they want us to figure out the area, the area of the surface of this candy. Or essentially the area of this circle. And so when we think about area, we know that the area of a circle, the area of a circle is equal to pi times the radius of the circle squared. Times the radius of the circle squared, and you say, well they gave us the diameter, what is the radius? Well, you might remember the radius is half of diameter, so distance from the center of the circle to the outside, to the boundary of the circle. So it would be this distance right over here, which is exactly half of the diameter. So, it would be eight millimeters. So, where we see the radius, we could put eight millimeters. So the area is going to be equal to pi times eight millimeters squared, which would be 64, 64 square, 64 square millimeters. And typically, this is written with pi after the 64, so you might often see it as this is equal to 64 pi, 64 pi millimeters squared, millimeters squared, millimeters squared. Now, this is the answer, 64 pi millimeters squared, but sometimes it's not so satisfying to just leave this pi, you might say, "Well, I wanna get a estimate of what number "this is close to, I wanna decimal representation of this." And so we can start to use approximate values of pi. So, the most rough approximate value that tends to be used is saying that pi, a very rough approximation is equal to 3.14. So in that case, we could say that this is going to be equal to 64, 64 times 3.14 millimeters, millimeters squared and we can get our calculator to figure out what this will be in decimal form. So we have 64 times 3.14 gives us 200.96. So we could say that the area is approximately equal to, approximately equal to 200.96 square millimeters. Now, if we wanna get a more accurate representation of this, pi actually just keeps going on and on and on forever, we could use the calculator's internal representation of pi. In which case we'll say 64 times and then we have to look for the pi in the calculator, it's up here in this yellow so I'll do this little second function, get the pi there, every calculator will be a little different. But 64 times pi, now we're going to use the calculator's internal approximation of pi, which is going to be more precise than what I had in the last one and you get 201, so let me put it over here so I can write it down, so a more precise is 201, and I'll round, I'll round to the nearest, I'll round to the nearest hundredth so you get 201.06, so 201, so more precise is 201.06 square millimeters. So this is closer to the actual answer 'cause a calculator's representation is more precise than this very rough approximation of what pi is.

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Area of a circle (video) | Geometry | Khan Academy

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Teacher] A candy machine creates small chocolate wafers in the shape of circular discs. The diameter, the diameter of each wafer is 16 millimeters. What is the area of each candy? So, the candy, they say it's the shape of circular discs. And they tell us that the diameter of each wafer is 16 millimeters.

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Something I have trouble with is circumference, and area.