**What is a circle? **

A circle is a locus of a point which moves in a plane, such that its distance from a fixed point in the plane is always constant.

The fixed point is called the centre of the circle and the constant is called its radius.

**The equation of the circle: **Its the set of all points lying on the circumference of the circle.

**Chord and diameter of the circle: **The line joining any two points on the circumference is called a chord. If any chord passing through its centre is called its diameter.

diameter d= 2r

## Equation of the circle in different forms

**(a). Centre- radius form : **

This equation is known as central form of the equation of a circle.

NOTE: when centre (0, 0) then the equation of the circle becomes

** **

which is known as the **standard form** of the circle.

**(b) Parametric form : **The parametric equation of the circle are

**(c) General form: **The equation of the circle with centre ( h, k) and radius r is

Which is of the form +2gx+2fy+c=0.

Relative positions of two circles:

Pole, polar , chord of contact, conjugate lines and conjugate points