ABSTRACT
The points in a plane may be placed in one-toone correspondence with pairs of real numbers. A common method is to use perpendicular lines that are horizontal and vertical and intersect at a point called the origin. These two lines constitute the coordinate axes; the horizontal line is the x-axis and the vertical line is the y-axis. The positive direction of the x-axis is to the right, whereas the positive direction of the y-axis is up. If P is a point in the plane, one may draw lines through it that are perpendicular to the x-and y-axes (such as the broken lines of Figure 4.1). The lines intersect the x-axis at a point with coordinate x1 and the y-axis at a point with coordinate y1. We call x1 the x-coordinate, or abscissa, and y1 is termed the y-coordinate, or ordinate, of the point P.
