4.1. INTRODUCTION

We know that the position of a point in a plane can be determined by two real numbers representing its distances from two fixed mutually perpendicular straight lines called the coordinate axes. In practical life, we cannot expect a point to always lie in a plane ; rather it lies in the three-dimensional space in which we are living. This necessitates the introduction of three-dimensional geometry in which the position of a point in space is determined by three real numbers.