A circle is a perfectly round geometric shape which you will see again and again in your studies of math concepts. You will need to have an understanding of its diameter, radius, circumference, and area.

All the points on a circle are at an equal distance from the center. If you think of a perfectly round pie as a circle, each point on the border represents an endpoint. When you slice a pie from one end of the border to the other, you form a chord. A chord is any line that passes from one endpoint of a circle to another. The diameter is the longest chord in a circle that divides the shape directly in half. The radius is a line that travels from the center of a circle to an endpoint. Two radii add up to become the diameter. On the ISEE, you should be able to determine the radius from the diameter and vice versa.

Imagine that you want to find the distance around the border of the pie or circle. This measurement is called the circumference. To find out how much space is inside of the border of pie, we could measure the area of the circle. The circumference and the area can be calculated by using the information provided about the radius or diameter.

- If the diameter of a circle is 9, what is the radius?
- If the radius of a circle is 7, what is the circumference?
- If the diameter of a circle is 12, what is the area?
- If the radius of a circle is 14, what is the circumference and area?
- If the area of a circle is 16π, what is the circumference?
- 16π
- 8π
- 64π
- 4π

**Answers to Practice Problems**

- 4.5
- 14π
- 36π
- 28π, 49π
- 8π

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