The general equation of the circle is

**x ^{2} + y^{2} + 2gx + 2fy + c = 0**

The equation represents a circle with

Centre = (−g, −f)

Radius = √(g^{2} + f^{2} − c)

The general second degree equation

ax^{2} + by^{2} + 2hxy + 2gx + 2fy + c = 0

represents a circle if

- a = b (coefficient of x
^{2}= coefficient of y^{2}) - h = 0 (no xy term)