The intersection of a sphere and the xy-plane is the circle $(x-1)^2+(y-2)^2=5$ and point $(1,2,1)$ is on the sphere The intersection of a sphere and the $xy$-plane is the circle $$(x-1)^2+(y-2)^2=5$$ and point $(1,2,1)$ is on the sphere. Find the center and radius of the sphere.
When the sphere and $xy$-plane intersect than $z=0$. 
I tried $(x-1)^2+(y-2)^2+(z+?)^2=5$ but what can I do next? 

One more :
Equation of the sphere:
$(x-a)^2+(y-b)^2+(z-c)^2=R^2.$
Cut with plane $z=0:$
$(x-a)^2+(y-b)^2 +c^2=R^2.$
1) Compare with the given circle:
$a=1,b=2, R^2-c^2= (√5)^2$
2) $(1,2,1)$ lies on the sphere:
$0+0+(1-c)^2=R^2.$
Hence: $1-2c+c^2=R^2;$
$1-2c= R^2-c^2=(√5)^2.$
$c= (1-5)/2=-2$, and $R^2 =9$.
