The general equation of the circle is
x2 + y2 + 2gx + 2fy + c = 0
The equation represents a circle with
Centre = (−g, −f)
Radius = √(g2 + f2 − c)
The general second degree equation
ax2 + by2 + 2hxy + 2gx + 2fy + c = 0
represents a circle if
- a = b (coefficient of x2 = coefficient of y2)
- h = 0 (no xy term)