The equation x2+y2+2gx+2fy+c=0x^2 + y^2 + 2gx + 2fy + c = 0x2+y2+2gx+2fy+c=0 represents a circle of radius 000 (a point) if:
g2+f2=cg^2 + f^2 = cg2+f2=c
g2+f2>cg^2 + f^2 > cg2+f2>c
g2+f2<cg^2 + f^2 < cg2+f2<c
g2+f2=0g^2 + f^2 = 0g2+f2=0