Perhaps you should try some spelling variations:
doc sach » hoc sach, toc sach, nuoc sach
nam thuc » nam thuoc, am thuc, nam thu
hieu ve » hieu va, hieu van
thuc va » thuc vat, thuc dan, thuc day
va hanh » van hanh, va thanh, va danh
vai cau » va cau, van cau, lai chau
vai cac » va cac, tai cac, voi cac
ve nam » he nam, va nam, le nam

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 y ((nang cao) or (nang x)) hoi. hieu ve nam thuc va hanh ((vai cau) OR (vai cac)) tren~.
• If you are trying to use Boolean operators, they must be ALL CAPS: doc sach y ((nang cao) OR (nang x)) hoi. hieu ve nam thuc va hanh ((vai cau) OR (vai cac)) tren.
• The words AND, OR and NOT may confuse the search; try adding quotes: "doc sach y ((nang cao) or (nang x)) hoi. hieu ve nam thuc va hanh ((vai cau) OR (vai cac)) tren".
• Adding a wildcard symbol may retrieve word variants: doc sach y ((nang cao) or (nang x)) hoi. hieu ve nam thuc va hanh ((vai cau) OR (vai cac)) tren*.