geometry
posted by Ray .
Center and radius of x^2 + y^2 – 4y = 0.

STD Form of Circle:
(x  h)^2 + (y  k)^2 = r^2, C(h , k).
x^2 + y^2  4y = o,
Complete the square:
x^2 + y^2  4y + (4 / 2)^2 = (4/2)^2,
x^2 + y^2  4y + 4 = 4,
Convert the perfect sq to a binomial:
(x  0 )^2 + (y  2)^2 = 4 = r^2, r=2.
C(0 , 2).