令圆心为(t,3t),半径为r（r>0）则圆的标准方程为(x-t)^2+(y-3t)^2=r^2联立直线y=x得2x^2-8tx+10t^2-r^2=0因直线与圆相切,则⊿=0即64t^2-8(10t^2-r^2)=0即r^2=2t^2（I）显然圆只与y轴相交令x=0,代入圆方程有y=3t±√(...