已知圆心A(x1,y1),切点(x2,y2)
则:半径r^2=(x1-x2)^2+(y1-y2)^2
圆方程: (x-x1)^2+(y-y1)^2=(x1-x2)^2+(y1-y2)^2
(切线方程,是个多余的条件)