1, perimeter = pi × diameter, letter formula: c = π d.

2. perimeter =? Pi × radius × 2, letter formula: c = 2π r.

The length of the curve forming a circle is the circumference of the circle. The circumference of a circle depends on its diameter (radius).

Pi refers to the ratio of circumference to diameter.

In a plane, a closed curve formed by a moving point rotating around a certain point with a certain length is called a circle. A circle has countless points.

On the same plane, the set of points whose distance to a fixed point is equal to a fixed length is called a circle. A circle can be expressed as a set {m |||| mo | = r}, and the standard equation of a circle is (x-a)? + (y - b)？ = r？ . Where o is the center of the circle and r is the radius.

A circle is a geometric figure. By definition, compasses are usually used to draw circles. The diameter and radius length 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. 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.

Extended data

The difference between semi-circular arc and semi-circular;

Semi-arc: Its length is half of the circumference.

Semicircle: Its length is half of the circumference, plus the length of a diameter.

Formula for calculating circle area:

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Find the diameter from the area of the circle:

Divide a circle into several equal parts and you can make an approximate rectangle. The width of a rectangle corresponds to the radius of a circle.