In a plane, a closed curve formed by rotating around a point for a certain length is called a circle. On the plane, a circle is a set of points whose distance from a fixed point is equal to a fixed length, which is called a circle. A circle has countless axes of symmetry, which pass through the center of the circle. A circle has rotation invariance.
A circle is a conic curve, which is obtained by cutting a cone from a plane parallel to the bottom of the cone. The circle is defined as 360, because the ancient Babylonians moved one position every 4 minutes and 360 positions every 24 hours, so the inner angle of the circle is defined as 360. This degree represents the sun.
A circle is an axisymmetric figure, and its axis of symmetry is an arbitrary straight line passing through the center of the circle. A circle is also a central symmetric figure, and its symmetric center is the center of the circle. Vertical diameter theorem: the diameter perpendicular to the chord bisects the chord and bisects the two arcs opposite the chord. Inverse theorem of vertical diameter theorem: bisecting the diameter of a chord (not the diameter) is perpendicular to the chord and bisecting two arcs opposite to the chord.
The properties and theorems of the angle of a circle and the central angle are in the same circle or the same circle. If the distance between two central angles, two central angles, two sets of arcs, two chords and one of the two chords to the center of the circle is equal, the other corresponding quantities are equal.
Introduction to history
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.