Enclose a sphere in a cylinder and cut out a spherical segment by slicing twice
perpendicularly to the cylinder's
axis. Then the lateral surface
area of the spherical segment is equal to the lateral surface
area cut out of the cylinder
by the same slicing planes, i.e.,
where
is the radius of the cylinder
(and tangent sphere) and
is the height of the cylindrical (and spherical) segment.