Perhaps you should try some spelling variations:
doc sach » hoc sach, toc sach, nuoc sach
kho tang » cao tang
nam thuc » nam thuoc, am thuc, nam thu
cau tren » cu tren, dau tren, ca tren
ky tang » ky nang, ky dang, ky trong
hieu va » lieu va, hieu qua
thuc ca » thuc cua, thuc can, thuc cao
dan van » dang van, doan van, dao van
van cau » van dau, van hau, van chat
va nam » van nam, ha nam, va nay
ca dan » cac dan, ca dao

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 tang) or (kho tang)) xa hoi. hieu va nam thuc ca dan van cau tren~.
• If you are trying to use Boolean operators, they must be ALL CAPS: doc sach ((ky tang) OR (kho tang)) xa hoi. hieu va nam thuc ca dan van cau tren.
• Adding a wildcard symbol may retrieve word variants: doc sach ((ky tang) or (kho tang)) xa hoi. hieu va nam thuc ca dan van cau tren*.