9.3.1 Introduction

Everybody has an intuitive understanding of what a circle is:
All points on the (red) circle have the same distance from exactly one point, namely the centre $M$. This distance $r$ is the so-called radius of the circle (see Module 5). If we now want to describe circles in a given coordinate system by equations, as we did in the previous section for lines, and use these descriptions for calculations, we will have to examine the meaning of the terms circle, centre, distance, and radius in more detail. This will enable us to specify an equation of a circle. This is the subject of this section.