Perhaps you should try some spelling variations:
toc sach » hoc sach, doc sach, thuoc sach
cap nuoc » dat nuoc
doan vi » doan va, don vi, doan thi
sach ky » sac ky, sach y, sach ty
hieu va » lieu va, hieu qua
dai hoi » dai hoc, dai hai, dai moi
lai hoi » lai hoa, lai roi, mai hoi
ky tang » ky nang, ky dang, kho tang
va cap » va cac, va dap, va cao
dan vi » don vi, dan va, gan vi

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