This is a question from my calculus 3 class.

"Find the equations for the following right circular cylinders. Each cylinder has a radiusaand is tangent to two coordinate planes."

Basically I am given three 3D graphs of cylinders tangent to the 3 different planes. The center points for the different graphs are (a, 0, a), (a, a, 0), and (0, a, a). Basically here is what I have for my cylinder equations:

(a, 0, a) ... ax^2 + az^2 = a

(a, a, 0) ... ax^2 + ay^2 = a

(0, a, a) ... ay^2 + az^2 = a

I am however a little confused as to whether I square the radius or not? Either way, am I doing it right? I feel something is wrong maybe.

I've attached a picture of the graphs in my textbook.