m = -5
圆心o坐标:（m,-2）.
设半径为r,则 r^2-m^2 = 7..(1)
设点o到直线的距离为d.则 r - d = 根号2.
即 r = |m-1|/根号2 + 根号2 .(2)
联立(1)(2),消r,分m>=1 和 m