#### Chapter 9 Objects in the Two-Dimensional Coordinate System

**Section 9.3 Circles in the Plane**

# 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.