A circle is a geometric figure. The diameter and radius of the inner circle of the same circle are always the same, and the circle has countless radii and diameters. A circle is a figure with axial symmetry and central symmetry.
The axis of symmetry is the straight line where the diameter lies. At the same time, the circle is a "positive infinite polygon" and "infinity" is just a concept. A circle can be regarded as a regular polygon composed of countless infinitesimal points. The more sides a polygon has, the closer its shape, perimeter and area are to a circle. So there is no real circle in the world, and the circle is actually just a conceptual figure.
Historical introduction of the circle:
1, 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.
2. 18 thousand years ago, cavemen in caves used to drill holes in animal teeth, gravel and stone beads. Some of those holes were very round. In the pottery age, many pottery were round.
3. 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.
I can make a circle, but I don't necessarily know its nature. The ancient Egyptians believed that the circle was a sacred figure given by God. It was not until more than two thousand years ago that China's Mozi (about 468- 376 BC) gave the definition of a circle: a circle, a circle of equal length.
6. It means that a circle has a center and the length from the center to the circumference is equal. This definition is 100 years earlier than that of the Greek mathematician Euclid (about 330 BC-275 BC).
Above content reference: Baidu Encyclopedia-Circle