过原点:x^2+y^2+ax+by=0
圆心在x轴上:(x-a)^2+y^2=r^2
圆心在y轴上:x^2+(y-a)^2=r^2
与x轴相切:(x-a)^2+(y-b)^2=b^2
与y轴相切:(x-a)^2+(y-b)^2=a^2
与两坐标轴都相切:(x+/-a)^2+(y+/-a)^2=a^2