Shares and common fractions

All points of a circle are the same distance from its centre; the part of the plane bounded by a circle is called a disc, and its diameter is 2 times its radius. If a whole thing is divided into equal pieces, each piece is called a share. For example, if a cake is cut into 8 equal pieces, each piece is «one eighth». If 3 of the 8 shares are taken, we get «three eighths», written 3/8. Such a notation is a common fraction: the number below the fraction line is the denominator (into how many shares the whole was divided), the number above is the numerator (how many shares were taken). A half is 1/2, a quarter is 1/4, an eighth is 1/8. To find a part of a number, divide the number by the denominator and multiply by the numerator.