What is Circle?

Also known as: diameter

Definition 26.6 Primary & Middle School Mathematics · Chapter 26 — Lines and Polygons

The circle of center $O$ and radius $3$ cm is the set of all points at exactly $3$ cm from $O$ . The compass draws it: spike on $O$ , opening $3$ cm, turn. A diameter is a segment through the center joining two points of the circle; it measures twice the radius.

Examples

Example 26.7 (The compass as a distance-keeper)

The compass does more than draw circles: it transports distances. To mark all points at $4$ cm from a point $A$ , draw the circle of center $A$ , radius $4$ cm; to compare two segments without a ruler, open the compass on one and test it on the other.

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Definition 40.5 Primary & Middle School Mathematics · Chapter 40 — Lines, Circles and Angles

The circle of center $O$ and radius $r$ is the set of all points at distance exactly $r$ from $O$ . A segment from the center to the circle is a radius; a segment joining two points of the circle through the center is a diameter — its length is $2r$ ; a segment joining two points of the circle is a chord.

A circle with a radius [OM], a diameter [AB] (a chord through the center, twice as long as the radius) and a chord [CD]. — A circle with a radius $[OM]$ , a diameter $[AB]$ (a chord through the center, twice as long as the radius) and a chord $[CD]$ .

Examples

Example 40.6

“Draw the circle of center $O$ passing through $A$ ”: open the compass from $O$ to $A$ — the radius is the distance $OA$ — and turn. Every point of this circle is at the same distance from $O$ as $A$ .

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Examples

Examples

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