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High School World History: Tutoring Solution35 chapters | 452 lessons

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Lesson Transcript

Instructor:
*Yuanxin (Amy) Yang Alcocer*

Amy has a master's degree in secondary education and has taught math at a public charter high school.

In this video, you'll learn how to define and find the circumference of a circle using two equations: one that involves the diameter and another that involves the radius.

Our earth is a sphere, and if we sliced it in half, we'd end up with a circle. **Circumference** is the distance around a circle. We can measure the circumference of the earth by measuring the distance we'd cover if we walked the way around the world. To understand circumference, we also need to understand the meaning of diameter and radius.

The **diameter** is the distance across the center of a circle, shown in green in this image. The **radius** is the distance from the center to the edge of a circle, shown in red.

There are two ways to write the equation for the circumference.

You can use either of these equations, depending on whether the problem provides you with the diameter or the radius.

There are two ways you can apply the equation for the circumference of a circle using the diameter.

The first is by using the equation to find the circumference given the diameter. For example, if a problem gives you a diameter of 7, you'd plug in the value of the diameter for *d* and replace pi with 3.14.

The second way to use the equation is when you need to find the diameter given a circumference, such as 21.98 inches. To do this, divide both sides by pi.

There are also two ways you can apply the equation for the circumference of a circle using the radius.

The first is by using the equation to find the circumference given the radius of a circle. For example, let's say the problem gives you a radius of 6 inches. You'd plug the 6 into the *r* variable and replace pi with 3.14.

The second way to use the equation is when you're asked to find the radius given a circumference, such as 37.68 inches. Here, you'll rely upon the equation for the circumference using the radius, but this time you'll use algebra to solve the equation for *r*. To do this divide both sides of the equation by 2 and pi.

In this lesson, you learned that **circumference** is the distance around a circle. You also learned that the **diameter** is the distance across the center of a circle and the **radius** is the distance from the center to the edge of a circle. There are two equations you can use to find the circumference of a circle. One involves the diameter and the other involves the radius.

Alternatively, you can also use these equations to find the diameter or radius of a circle when given the circumference.

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3 in chapter 2 of the course:

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High School World History: Tutoring Solution35 chapters | 452 lessons

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