Perhaps you should try some spelling variations:
toc sach » hoc sach, doc sach, thuoc sach
cap nuoc » dat nuoc
hieu van » hieu va, thien van, lieu va
ky tang » ky nang, ky dang, kho tang
mai hoi » mai khoi, dai hoi, lai hoi
van cap » van cau, van dap, van ca
ky sang » ky nang, ky dang, duy sang
dan vi » doan vi, don vi, gan vi
dan va » doan va, dan ca, ban va

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

• Performing a fuzzy search may retrieve terms with similar spellings: toc sach ((ky sang) or (ky tang)) mai hoi. hieu van cap nuoc va ((dan vi) OR (dan va)) cau tao~.
• If you are trying to use Boolean operators, they must be ALL CAPS: toc sach ((ky sang) OR (ky tang)) mai hoi. hieu van cap nuoc va ((dan vi) OR (dan va)) cau tao.
• The words AND, OR and NOT may confuse the search; try adding quotes: "toc sach ((ky sang) or (ky tang)) mai hoi. hieu van cap nuoc va ((dan vi) OR (dan va)) cau tao".
• Adding a wildcard symbol may retrieve word variants: toc sach ((ky sang) or (ky tang)) mai hoi. hieu van cap nuoc va ((dan vi) OR (dan va)) cau tao*.