Perhaps you should try some spelling variations:
toc sach » hoc sach, thuoc sach, nuoc sach
nam thuc » nam thuoc, am thuc, nam thu
hieu van » hieu va, thien van, lieu va
dau treo » dau tren, cau treo, dau theo
doc sach » hoc sach, nuoc sach, dac sac
ky lang » ky nang, ky dang, ky tang
lang xa » lang ha, dang xay, lang van

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) or (doc sach)) ky lang xa hoi. hieu van nam thuc ((va dao) OR (ca dao)) va dau treo~.
• If you are trying to use Boolean operators, they must be ALL CAPS: ((toc sach) OR (doc sach)) ky lang xa hoi. hieu van nam thuc ((va dao) OR (ca dao)) va dau treo.
• The words AND, OR and NOT may confuse the search; try adding quotes: "((toc sach) or (doc sach)) ky lang xa hoi. hieu van nam thuc ((va dao) OR (ca dao)) va dau treo".
• Adding a wildcard symbol may retrieve word variants: ((toc sach) or (doc sach)) ky lang xa hoi. hieu van nam thuc ((va dao) OR (ca dao)) va dau treo*.