1. Introduction to Circles

2. What is a circle?
A circle is defined as the path traced by a point which moves at a constant distance from a fixed point on a given plane surface. The fixed point is called the centre of the circle. In the figure, point O is the centre of the circle. O

3. Radius
The radius is a line segment with one end point at the centre and the other end on the circle. It is denoted by ‘r’. In the figure, OA is the radius. A
Radius(r) O

4. Chord
A line segment joining any two points on the circle is called a chord. In the figure, AB and CD are chords. D
Chord C
Chord A B

5. Diameter
Diameter is a chord passing through the centre of the circle. It is denoted by ‘d’. In the figure, AB is the diameter. The diameter is the longest chord. It is twice the radius i.e. d = 2r The diameter divides the circle into two equal parts. Each equal part is a semicircle.
Diameter(d) A B O

6. Circumference
The distance around a circle is called the circumference of the circle, and is denoted by ‘C’. i.e., The perimeter of a circle is known as its circumference.
Circumference(C)