1. The problem statement, all variables and given/known data Describe in words the region of R3 represented by the inequality x^2 + z^2 <= 9 2. Relevant equations Equation of a sphere= (x-h)^2 + (y-k)^2 + (z-l)^2 = r^2 3. The attempt at a solution Since there is no y value in the given inequality, I stated that it would be points in or on a circle on the xz-plane with center at the origin, and the radius is 3 with respect to the xy-plane. However, my book says this inequality describes a cylinder of radius 3 with y-axis. Can someone explain this to me please? How can it be a cylinder? And why is the radius with the y-axis and not with the xy-plane?