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Seek the knowledge points of this chapter in the third grade mathematics circle.
Characteristics of circle: A circle is a closed figure composed of a curve, and the distance from any point on the circle to the center of the circle is equal.

The function of center and radius: the center determines the position of the circle, and the radius determines the size of the circle.

A circle is an axisymmetric figure, and a straight line centered on the diameter is the symmetry axis of the circle. A circle has countless axes of symmetry.

The diameter of the same circle is twice the radius.

The circumference of a circle refers to the length of the curve surrounding the circle. The circle with large diameter grows up, and the circle with small diameter has small circumference.

The quotient of the circumference divided by the diameter of the circle is a fixed number. We call it pi, which is usually 3. 14 in calculation.

Circumference: C=2πr or c = π d.

Find the radius: r=C/2π

Find the diameter: d=C/π

The area meaning of a circle: the plane size or the surface size of a circular object or figure is the area of a circle.

Area calculation formula: the square of π * R.

Calculation method of circular area: S=πR square -πr square or S=π(R square -r square).

(R is the radius of the big circle and R is the radius of the small circle)