In a plane, a moving point with a certain point as the center and a certain length as the distance to form a closed curve is called a circle. A circle has countless axes of symmetry.

In the same plane, the set of points whose distance from a fixed
point is equal to a fixed length is called a circle.
A circle can be expressed as a set {m|Mo|=R}, where o is the
center of the circle and R is the radius. The standard equation of
a circle is (x - a)^{2} + (Y - b)^{2} = R^{2}, where the point (a, b) is
the center of the circle and R is the radius. A circle is a kind of
conic curve, which is obtained by cutting a cone parallel to the
bottom of the cone.

A circle is a geometric figure. By definition, circles are usually drawn with compasses. The diameter and radius length of the inner circle of the same circle are always the same. A circle is an axisymmetric and centrosymmetric figure. The axis of symmetry is the line where the diameter lies. At the same time, circle is "positive infinite polygon", while "infinity" is only a concept. When the polygon has more sides, its shape, perimeter and area are closer to the circle. Therefore, there is no real circle in the world. The circle is actually a conceptual figure.

Here, the area of the circle does not include the calculation of the cylinder, only the surface area of the circle.

π---circumferential rate S---area L---circumference r---circle radius d----circle diameter

The formula for calculating the area of the circle: S = π×r2 =3.1416×r2

The formula for calculating the circumference: L = 2×π×r

(The area of the circle is a bit white: the radius is multiplied by the radius by 3.14)

Know the diameter of the circle area: diameter: 2 √ (area ÷ pi)

Example of area: a single cable with a diameter of 80 mm, and the cross-sectional area

3.14×(40×40) or 3.14×402

= 3.14 × 1600 = 5024 (mm^{2})

Calculate the volume calculation formula for the ball: 4.18879×r3

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