A circle is the locus of a point which moves in a plane in such a way that its distance from the fixed point in the same plane is always constant.
The fixed point is called the centre of the circle and the constant distance is called the radius of the circle.
Standard Equation of the Circle
Let C(h, k) be the centre of circle having radius a. Now, let P(x, y) be any general point such that |PC|=a always
Squaring both sides, we have
which is the equation of the circle in central form.