D determines that the abscissa of the center of the circle is -D/2, and if D=0, the center of the circle is on the X axis.

E determines that the ordinate of the center of the circle is -E/2. If E=0, the center of the circle is on the Y axis.

The radius of DEF is (d 2+e 2-4f)/2, if D2+e 2-4f >; 0, the equation represents a circle.

If d 2+e 2-4f = 0, the equation only represents one point (-D/2, -E/2).

If d 2+e 2-4f