ABSTRACT
A method of identifying each element of a set by a number or by a pair of numbers or even by a set of numbers is called a coordinate system. The plane R2, which consists of all pairs of real numbers (x, y), comes to us with a ready made coordinate system, the Cartesian coordinate system. Sets admit, however, many different coordinate systems. A point in R2 can also be identified by its distance from the origin, r, and the angle, θ, that the line joining it to the origin makes with the positive x-axis (Figure 21.1).
