Explanation:
the surface area of a half-cylinder is, surprise, surprise, half of the surface area of a whole cylinder ...
the surface area of a cylinder is
A=2×pi×r×h + 2×pi×r²
r being the radius, h being the height of the cylinder.
the formula simply adds
the sidewall area (which is a rectangle of
circle circumference × height)
+
the top and base circle areas
half of this area calculation for a half-cylinder is
A/2 = pi×r×h + pi×r²