Some one help me, please. I do not understand the problem.

Show that for any numbers r, a, and h so that r, h > 0 and that the circular strip formed by truncating a sphere of radius r between x = a and x = a+h has the same surface area regardless of a. I.e, show that this value is not dependent on a.

In my opinion, the area of the circular strip is largest when it is on the middle of the sphere and it is smallest at top. But it sounds wrong when the question is about proving they are all equal.