A circle is a curve bounding a plane, called the circle. In other words, the circumference is called the locus of points in the plane equidistant from the centre. Segments connected to a center of a circle are called its radii, and the distance from one point to another passing through the centre is called a diameter of the circle. The diameter of a circle equal to twice the radius: D=2r. The equation of a circle in analytic geometry is:x^2+y^2=R^2Существует also the concept of the chord of the circle. It is also obtained by connecting two points of the circle, but not necessarily passes through its center. All the diameters that pass through the middle of the chord perpendicular to it. The biggest chord of a circle is its diameter.
As with any curve, a circle has a certain length p. It has long been observed that the length of a circle relates to its diameter the number PI:p/d=patsada it follows that p=πd, or p = 2nr, where r is the radius of the circle.The number π is an irrational value, but it is approximately equal to 3.14.Knowing the length of the circumference, and determine the area of the ring bounded by two circles. It is equal to:S=2nr*k, where k is the distance between the inner and outer circumferences of the ring; 2nr - length internal circumference of the ring.
Graphical way to determine the length of the circumference, because of its inaccuracies, is rarely used. For this purpose an odometer is a device for measuring the length of a curve. Anywhere circles mark the start point of the measurement. To bring her odometer, and lead him along the line until it gets to the same point.
The definition of the length of the circumference has considerable practical value. In addition to mathematicians, it is necessary to carry out physicists, astronomers. First calculate the length of the circumference of elementary particles, the second of the heavenly bodies. Also, knowing the diameter of the circus arena, a running track, guided by the above formulas, we can calculate what distance they will run a horse or runner at a range.