Question

If two solid-hemisphere s of same base radius r are joined together along their bases , then curved surface area of this new solid is
(a) $4{\mathrm{\pi r}}^2$ (b) $6{\mathrm{\pi r}}^2$ (c) $3{\mathrm{\pi r}}^2$ (d) $8{\mathrm{\pi r}}^2$

Answer 1

Base radius of the hemisphere = r
Since the two hemispheres are joined end to end, it becomes a complete sphere.
Curved surface area of the new solid = total surface area of the sphere.
$\mathrm{Curved}\mathrm{surface}\mathrm{area}\mathrm{of}\mathrm{the}\mathrm{new}\mathrm{solid}=4{\mathrm{\pi r}}^2$

Hence, the correct answer is option (a).