x^2+y^2+dx+ey+f=0
(x+d/2)^2+(y+e/2)^2=1/4(d^2+e^2)-f
当1/4(d^2+e^2)-f＞0时,表示以（-d/2,e/2)为圆心,在√[1/4(d^2+e^2)-f]为半径的圆