## Circumference of a Circle

### Circumference: Learn

The circumference of a circle is the distance around the outside of the circle. It could be called the perimeter of the circle.

When you start to learn about circles, you begin to work with the constant number pi (π), which represents the ratio of the circumference to the diameter. That is, if you could perfectly measure every circle's diameter and circumference, then pi is what you get when you divide the circumference by the diameter.

We can use that idea in reverse to find the circumference when all we know is the diameter. The value of the number pi is irrational (the decimal has no pattern and never ends), so we estimate pi by rounding it to 3.14.

How to find the circumference of a circle:

- Multiply pi ( π = 3.14 ) by the diameter of the circle.
- Example: If a circle has a diameter of 4, its circumference is 3.14*4=12.56
- Tip: If you know the radius, the diameter is twice as large.

### Perimeter: Practice

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What is the circumference of a circle whose

is cm?

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