The Cartesian product R 3 R 3 R − hs x , y , z d | x , y , z [ R j is the set of all ordered triples of real numbers and is denoted by R 3 . We have given a one-to-one correspon- dence between points P in space and ordered triples sa, b, cd in R 3 . It is called a three- dimensional rectangular coordinate system. Notice that, in terms of coordinates, the first octant can be described as the set of points whose coordinates are all positive.