Such geometrical figure as circle, has several characteristics: radius, diameter, area, circle length. All of them are interconnected among themselves. It means that enough information for definition of all other characteristics of the same circle is put into any of them.

## Instruction

1. The circle represents the curve limiting the plane called a circle. In other words, a circle is called the locus of the plane, equidistanted from its center. The pieces connected to the center, circles are called as its radiuses, and the distance from one point to another passing through the center is called diameter of a circle. Diameter of a circle is equal to two radiuses: D=2r. The circle equation in analytical geometry has an appearance: x^2+y^2=R^2Существует also concept of a chord of a circle. It turns out by connection of two points of a circle too, but not necessarily passes through its center. All diameters which pass through the middle of a chord are perpendicular to it. The biggest chord of a circle is its diameter.

2. As well as any curve, a circle has some length of p. Long ago it is noticed that length of a circle is connected with its diameter number π:p/d=πОтсюда follows that p=πd, or p = 2πr where r is circle radius. The number π has irrational value, but approximately it is equal to 3.14. Knowing circle length, it is possible and to determine the area of the ring limited to two circles. It is equal: S=2πr*k, where k - distance between internal and external circles of a ring; 2πr - length of an internal circle of a ring.

3. The graphic way of determination of length of a circle, because of its inaccuracy, is used seldom. For this purpose use the curvimeter - the device for measurement of length of a curve. In any place of a circle note the initial point of measurement. To it bring the curvimeter and conduct it on the line until again hit the same nail.

4. physicists, astronomers should carry out. The first calculate length of a circle of elementary particles, the second - celestial bodies. Also, knowing diameter of the circus arena, the racetrack, being guided by the formulas given above, it is possible to calculate what distance will be run by a horse or the runner for one circle.