Let XOX′ and YOY′ be two fixed perpendicular lines in the plane of the paper. The line OX is called the axis of X and OY the axis of Y. OX and OY together are called the coordinate axes. The point O is called the origin of the coordinate axes. Let P be a point in this plane. Draw PM perpendicular to XOX′. The distance OM is called the x-coordinate or abscissa and the distance MP is called the y-coordinate or ordinate of the point P.
If OM = x and MP = y then (x, y) are called the coordinates of the point P. The coordinates of the origin O are (0, 0). The lines XOX′ and YOY′ divide the plane into ...