The human brain has remarkable property – it removes that information which actively is not used by us on "boondocks". Therefore even the simplest mathematical rules and formulas learned at school periodically should be refreshed in memory. And if they are not there yet – to load into it. Also the formula of finding of length of a circle is among such data.

## Instruction

1. Length of a circle is, in fact, its perimeter, that is the sum of lengths of all parties. But as the concept "party" is not applicable to a circle (it represents the one and only curve which all points are equidistant from the center), length of all figure is calculated.

2. This size is designated by a letter which called all circle and its center is designated. For finding of its size it is necessary to know what radius (R) at a circle or diameter (D=2R). Diameter of a circle increased by number "пи" as a result gives the required **length** of a circle. The same result is yielded by the same number increased by two and at a radius size (as radius is twice less than diameter).

3. The number "пи" incorporates a huge number of figures. We for calculations will need its value rounded to hundredth parts – 3.14.

4. The result of calculations registers in centimeters or in those sizes in which the radius or diameter was given.

5. If you need to find length not of all circle, and only its part, then for calculations it is required, except radius, the size of a corner which top is in the center of a circle, and the parties limit the measured arch (this parameter is given in radians). To find arch length, increase this number by radius.