A circle is an important shape in the field of geometry. Let's look at the definition of a circle and its parts. We will also examine the relationship between the circle and the plane.

A?**circle**?is a shape with all points the same distance from its center. A circle is named by its center. Thus, the circle to the right is called circle A since its center is at?point?A. Some real world examples of a circle are a wheel, a dinner plate and?(the surface of) a coin.

The distance across a circle through the center is called the?**diameter**. A real-world example of diameter is a 9-inch plate.

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The?**radius**?of a circle is the distance from the center of a circle to any point on the circle. If you place two?radii?end-to-end in a circle, you would have the same length as one diameter. Thus, the diameter of a circle is twice as long as the radius.?

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We can look at a pizza pie to find real-world examples of diameter and radius. Look at the pizza to the right which has been sliced into 8 equal parts through its center. A radius is formed by making a straight cut from the center to a point on the circle. A straight cut made from a point on the circle, continuing through its center to another point on the circle, is a diameter. As you can see, a circle has many different radii and diameters, each passing through its center.?

A?**chord**?is a?line segment?that joins two points on a curve. In geometry, a chord is often used to describe a line segment joining two endpoints that lie on a circle. The circle to the right contains?**chord AB**. If this circle was a pizza pie, you could cut off a piece of pizza along chord AB. By cutting along chord AB, you are cutting off a segment of pizza that includes this chord.

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It turns out that a diameter of a circle is the longest chord of that circle since it passes through the center. A diameter satisfies the definition of a chord, however, a chord is not necessarily a diameter. This is because every diameter passes through the center of a circle, but some chords do not pass through the center. Thus, it can be stated,?**every diameter is a chord, but not every chord is a diameter**.

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Let's revisit the definition of a circle. A?**circle**?is the set of points that are equidistant from a special point in the plane. The special point is the center. In the circle to the right, the center is point A. Thus we have circle A.

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A?**plane**?is a flat surface that extends without end in all directions. In the diagram to the right, Plane P contains points A, B and C.

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Can you think of some real world objects that satisfy the definition of a plane? At this level of mathematics, that is difficult to do. Intuitively, a plane may be visualized as a flat infinite sheet of paper. The top of your desk and a chalkboard are objects which can be used to represent a plane, although they do not satisfy the definition above.

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A circle divides the?**plane**?into three parts:

- the points INSIDE the circle
- the points OUTSIDE the circle
- and the points ON the circle

You can see an interactive demonstration of this by placing your mouse over the three items below.

A circle divides a plane into three parts:

- the points?
**INSIDE?**the circle? - the points?
**OUTSIDE?**the circle? - and the points?
**ON?**the circle?

Example 1:?

Name the center of this circle.

Answer:?Point B

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Example 2:

Name two chords on this circle that are not diameters.

Answer:?DE?and?FG

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Example 3:

Name all radii on this circle.

Answer:?BA,?BC,?BD?and?BG

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Example 4:

What are?AC?and?DG?

Answer:?AC?and?DG?are diameters.

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Example 5:

If?DG?is 5 inches long, then how long is?DB?

Solution:?The diameter of a circle is twice as long as the radius.

5 inches?**÷**?2 = 2.5 inches

Answer:?The length of?DB?is 2.5 inches

Summary:?A circle is a shape with all points the same distance from its center. A circle is named by its center. The parts of a circle include a radius, diameter and a chord. All diameters are chords, but not all chords are diameters. A plane is a flat surface that extends without end in all directions. A circle divides the plane into three parts: The points inside the circle, the points outside the circle and the points on the circle.

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**1. ?Which of the following is a chord, but not a diameter?**

**2. ?Which of the following is a radius?**

**3. ?Name the center of this circle.**

**4. ?What is?PR?(or?PQR)?**

**5. ?If?PQ?is 3 cm long, then how long is?PR?**