1. Standard Equation of Circle:

Centre = (0, 0) and

Radius = r

2. Equation of a circle in centre radius form:

Centre = (h, k), Radius = r.

3. Equation of a circle in General form:

Centre = (–g, –f )

r = g + f – c

4. Equation of a circle with points P(x , y ) and Q(x , y ) as extremities of diameter is

(x – x ) (x – x ) + (y – y ) (y – y ) = 0

5. Equation of circle through three non-collinear points P(x , y ), Q(x , y ) and R(x , y ) is: