L:y=1C:(x-1)^2+(y-2)^2=1C(1,2),r=1y=1与圆C的切点A(1,1),所求圆的圆心C2在直线:x=1上OA的中点B(0.5,0.5),OA的垂直平分线BCk(BC)=-1/k(OA)=-1BC:y-0.5=-(x-0.5)y-0.5=-(1-0.5)y=0C2(1,0),r=1(x-1)^2+y^2=1