Chapter 9 Objects in the Two-Dimensional Coordinate System Section 9.3 Circles in the Plane
Everybody has an intuitive understanding of what a circle is:
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.