# Geometry: Basic Terms

## Basic Terms

Start with a point C, and collect all of the points a fixed distance r units away from it. Give this collection of points a name: *circle*. I have drawn a circle in Figure 17.1. The starting point is called the *center* of the circle. Any line segment having the center of the circle as one endpoint and any point on the circle as the other endpoint is called a *radius* of the circle. Because all points on the circle are a distance of r units away from the center, all radii of a circle are congruent. This will be stated as a theorem, though the proof would take no more than a line or two, with the reasons being either “given” or “definition of a circle.”

**Theorem 17.1**: All radii of a circle are congruent.

##### Solid Facts

A **circle** is the set of all points in a plane that are a fixed distance from a given point.

The **center** of the circle is the point equidistant from all points on the circle.

A **radius** of a circle is a line segment with one endpoint being the center of the circle, the other endpoint being a point on the circle.

Don't limit yourself to only drawing radii of circles. Circles get really interesting when you connect points on a circle. A line segment that joins two points on the circle is called a *chord* of the circle. A *diameter* of a circle is a chord that contains the center of the circle. The length of a diameter of a circle is twice the length of the radius of a circle. This can be proven by using the Segment Addition Postulate (Postulate 3.5).

If you have three points on a circle, you can connect them to form an inscribed angle. An *inscribed angle* of a circle is an angle whose vertex is a point on the circle and whose sides are chords of the circle. You can also construct *inscribed polygons* by using points on the circle as the vertices.

Two circles that coincide are congruent. In order for two circles to be congruent, the lengths of the radii must be congruent. If two circles have the same center they are called *concentric circles*.

There are times when you will need to measure the distance around the circle. When you did this with polygons, it was called the perimeter. With circles, it will be called the *circumference* of the circle.

##### Solid Facts

A **chord** is a line segment that joins two points on a circle.

A **diameter** of a circle is a chord that contains the center of the circle.

An **inscribed angle** of a circle is an angle whose vertex is a point on the circle and whose sides are chords of the circle.

An **inscribed polygon** of a circle is a polygon whose vertices are points on the circle and whose sides are chords of the circle.

**Congruent circles** are circles that have congruent radii.

**Concentric circles** are circles that have the same center.

The **circumference** of a circle is the linear measure of the distance around the circle.

Excerpted from The Complete Idiot's Guide to Geometry © 2004 by Denise Szecsei, Ph.D.. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with **Alpha Books**, a member of Penguin Group (USA) Inc.

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