Perhaps you should try some spelling variations:
toc sach » hoc sach, doc sach, thuoc sach
nam thuc » nam thuoc, am thuc, nam thu
hieu van » hieu va, thien van, lieu va
lang ha » long ha
lang xa » dang xay, lang van
sach ky » sac ky, sach y, sach ty
thuc va » thuc vat, thuc dan, thuc day
vai cua » dai cua, hai cua, lai cua
va dan » ca dan, va dao, va dau

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