9.1. Points

An ordered pairs of real numbers may be used to describe a point in the coordinate system. There is a one to one correspondence between an ordered pair and a point in the plane. The graph of an ordered pair is a point (drawn as a dot) in the x-y plane. By convention, the first number is the distance along the x-axis; the second number is the distance along the y-axis. This lets us define a point P with an ordered pair {a, b}. The real number a is the x-coordinate or abscissa; b is the y-coordinate or ordinate.

Points are often named by using capital letters. The point {0, 0} is the origin of the graph sometimes identified by the letter O. In figure 9-1, point A corresponds to the ordered pair {1, 3}; point B corresponds to {-2, 2}.

Fig. 9-1: Cartesian plane