Surface area of sphere portion = 2πrh is a well known equation. It works for ANY portion of height h on a sphere, not just starting at the top. Of course that's also the surface area of the enclosing cylinder of radius r, which is how it's easy to remember this formula.
So , what is the thread ? Review of the calculation or prove the spherical cap area Quod Erat Demonstrandum ? If so yes.. good job.
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He is like a man building a house, who dug deep and laid the foundation on the rock. And when the flood arose, the stream beat vehemently against that house, and could not shake it, for it was founded on the rock..