Perhaps you should try some spelling variations:
doc sach » hoc sach, toc sach, nuoc sach
hieu van » hieu va, thien van, lieu va
cac nuoc » cap nuoc, hoc nuoc, dat nuoc
ky sang » ky nang, ky dang, duy sang
ky tang » ky nang, ky dang, kho tang
van cac » van hac, van cau, van bac
ma hoi » mai hoi, xa hoi, ma hoa
va dan » ca dan, va dao, va dau
dan vi » doan vi, don vi, dan va
vi cau » va cau, ve cau, di cau

You may be able to get more results by adjusting your search query.

• Performing a fuzzy search may retrieve terms with similar spellings: doc sach ((ky sang) or (ky tang)) ma hoi. hieu van cac nuoc va dan vi cau tao~.
• If you are trying to use Boolean operators, they must be ALL CAPS: doc sach ((ky sang) OR (ky tang)) ma hoi. hieu van cac nuoc va dan vi cau tao.
• Adding a wildcard symbol may retrieve word variants: doc sach ((ky sang) or (ky tang)) ma hoi. hieu van cac nuoc va dan vi cau tao*.