Tài liệu tham khảo WikiPedia: Vỏ_cầu http://mathworld.wolfram.com/SphericalShell.html https://books.google.com/books?id=xh48DQAAQBAJ&lpg... https://web.archive.org/web/20160802000613/http://... https://web.archive.org/web/20171220215141/https:/...