# Circumference

Calculating the circumference of a circle helps in many ways. Suppose you were building a circular pool with a diameter of 30 feet, and you want to put a decorative border around the outside edge. You'd need to know the length of the border material required to go around the entire pool. To find this, you would need to calculate the circumference of the circle. Fortunately, calculating circumference is easy, and we can do it in just a few steps.

## Why you need pi to calculate circumference

First, you need pi. No, we're not talking about dessert -- we're talking about the ratio of a circle's circumference to its diameter. Fortunately, we don't need to actually calculate pi, since ancient cultures such as the Egyptians and Greeks already accomplished this thousands of years ago. The value of pi is approximately 3.14259 -- although the number contains an infinite number of non-repeating digits!

The formula is simple: C = 2πr

Where "C" represents the circumference, π represents pi, and "r" represents the radius.

You could also write the formula like this: C = πd

Where "d" represents the diameter of a circle.

**Example 1:**

A bicycle tire has a diameter of 26 inches. What is the circumference of the tire?

To solve this problem, we can use the formula for the circumference of a circle, which is C = πd, where C is the circumference, π (pi) is a mathematical constant approximately equal to 3.14, and d is the diameter of the circle.

We are given that the diameter of the bicycle tire is 26 inches, so we can substitute this value into the formula:

C = πd, C = 3.14 * 26, C = 81.7 inches

Therefore, the circumference of the bicycle tire is approximately 81.7 inches.

**Example 2:**

A circular swimming pool has a diameter of 30 feet. The pool has a walkway around it that is 5 feet wide. What is the total area of the walkway? What is the total circumference of the pool and the walkway?

To solve this problem, we can start by finding "r" the radius of the pool, which is half of the diameter:

r = d/2 = 30/2 = 15 feet

Next, we can find the radius of the pool and the walkway together "R", by adding the 5 feet of the walkway to the radius of the pool:

R = r + 5 = 15 + 5 = 20 feet

Finally, we can find the total circumference of the pool and the walkway together by adding the circumference of the larger circle with radius R to the circumference of the pool with radius r:

Circumference of larger circle: 2πR = 2 * 3.14 * 20 ≈ 125.7 feet

Circumference of pool: 2πr = 2 * 3.14 * 15 ≈ 94.2 feet

Total circumference of pool and walkway is equal to the Circumference of the larger circle plus the Circumference of the pool which is approximately: 125.7 + 94.2 ≈ 219.9 feet

## Topics related to the Circumference

## Flashcards covering the Circumference

Common Core: 7th Grade Math Flashcards

## Practice tests covering the Circumference

MAP 7th Grade Math Practice Tests

Common Core: 7th Grade Math Diagnostic Tests

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