Definition 1: The set of points whose distance from a fixed point is equal to a fixed length is called a circle. This fixed point is called the center of the circle.

Definition 2: All points whose distance from a fixed point is equal to a fixed length are on the graph, and the distance from a point to a fixed point on the graph is equal to a fixed length.

In a plane, a closed curve formed by a moving point rotating around a certain point with a certain length is called a circle. A circle has countless axes of symmetry.

On the same plane, the set of points whose distance to a fixed point is equal to a fixed length is called a circle. A circle can be expressed as a set {m |||| mo | = r}, and the standard equation of a circle is (x-a)? + (y - b)？ = r？ . Where o is the center of the circle and r is the radius.

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History of the circle

A circle is a seemingly simple shape, but in fact it is very wonderful. The ancients first got the concept of circle from the sun and the moon on the fifteenth day of the lunar calendar. On the Neanderthal 18000 years ago, he used to drill holes in animal teeth, gravel and stone beads, some of which were very round. In the pottery age, many pottery were round. Round pottery is made by putting clay on a turntable.

When people start spinning, they make round stone spindles or ceramic spindles. The ancients also found it easier to roll when carrying logs. Later, when they were carrying heavy objects, they put some logs under big trees and stones and rolled them around, which was of course much more labor-saving than carrying them.

About 6000 years ago, Mesopotamia made the world's first wheel-a round board. About 4000 years ago, people fixed round boards under wooden frames, which was the original car.

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