When a line AB rotate by 360 degree taking center A, thus obtained figure is called circle.
A is called center of circle. A to any point on the circle joining is called radius, a line passing through the center of the circle is called diameter of circle and its value is 2 * radius.
The ratio between circumference to diameter is always constant and denoted by \pi its value is \frac{22}{7} or 3.14 approximately.

\frac{pI}/D= \pi
so circumference of circle (p)=\pi*D=2*\pi*r
Area of circle=\pi*r^2=pi*\frac{d^2}{4}
We divide a circle in to many small sectors, thus the length of an arc is reached near about the length of chord. Then merge them as below we have obtained a ractangle.

length of Chord

Length of chord=\frac{a^2+h^2}{2h}
where 2a=lengh of chord
h= height of arc
r=radius of circle

length of arc= arc/oce=Angle in degree/360
Radion: If are is equal in length to the radious, then the angle for form are called radion.

Segment of a circle – chord AB of a circle divides the circular region into two part, each part is called a segment of the circle.

Sector of the circle– the area surrounded by an arc and the two radius joining the end point of the arc with the center is called a sector of the circle.

Concentric circle– two or more circles with the given centre are called concentric circle.