If you notice,the second’s hand goes round the dial of the clock and its tip moves in a round path. This path traced by the tip of the second’s hand is called a Circle.The figure above represents a circle.’A’ lies inside the circle,B lies outside the circle and C lies on the circle.
The circle has various interesting properties about which you can understand from Reference Notes.1. If you understand better by videos,Video Lecture 1 covers all the topics,proofs and questions.
Optional : To understand the proof of the theorem that only one unique circle passes through three non-collinear points,refer to Video lecture 2.