Answer
Radius:$r=2$ and center: $(a,b)=(0,2)$
Work Step by Step
Use standard form of equation of circles; $(x-a)^2+(y-b)^2=r^2$
Here, $r$ defines radius and $(a,b)$ defines center.
Then, $x^2+y^2-4y=0$
or, $x^2+(y^2-4y)=0$
or, $(x-0)^2+(y^2-4y+4)=4$
or, $(x-0)^2+(y-2)^2=2^2$
Thus, on comparing the above equation with the standard form, we have
Radius:$r=2$ and center: $(a,b)=(0,2)$
