Definition of a circle:

A figure composed of all points on the (1) plane whose distance from a fixed point is equal to a fixed length is called a circle.

(2) A line segment on the plane rotates 360 around one end, leaving a track called a circle.

Center of circle:

(1) As defined in (1), the fixed point is the center of the circle.

(2) As defined in (2), the end point of the winding is the center of the circle.

(3) The intersection of any two symmetrical axes of a circle is the center of the circle.

(4) The bisector of the line segment perpendicular to any chord in the circle with two ends on the circle is the center of the circle.

Diameter and radius of the center of the circle:

Diameter: The line segment whose two ends pass through the center of the circle is called the diameter of the circle. The diameter is usually indicated by the letter d.

Radius: The line segment connecting the center of the circle and any point on the circle is called the radius of the circle. Radius is generally represented by the letter R.

There are countless circles in diameter and radius. The circle is an axisymmetric figure, and the straight lines of each diameter are the symmetry axis of the circle. In the same or equal circle: the diameter is twice the radius, and the radius is half the diameter. D=2r or r= half d. ..

Formula of circumference and area of a circle:

Circumference of the circle:

The length of the curve forming a circle is called the circumference of the circle, which is represented by the letter C. The ratio of the circumference to the diameter of the circle is called pi. The quotient of the circumference divided by the diameter of the circle is a fixed number called pi. It is an infinite cycle decimal (irrational number), represented by the letter π.

When calculating, we usually take its approximate value, π≈3. 14. The circumferential angle of the diameter is a right angle. A chord with a circumferential angle of 90 is a diameter.

Area formula of circle:

The size of the plane occupied by a circle is called the area of the circle. π r 2, represented by the letter s, the angle of the circle subtended by the arc is half of the central angle. In the same circle or in the same circle, the arcs with equal central angles are equal, the chords are equal, and the distances between chords are equal.

In the same circle or equal circle, if two arcs are equal, then their central angles are equal, their chords are equal and their chords are equidistant. In the same circle or equal circle, if two chords are equal, then their central angles are equal, their arcs are equal, and the distances between chords are equal.