Perhaps you should try some spelling variations:
cac thuc » cach thuc, cac thu, can thuc
hieu van » hieu va, thien van, lieu va
so sach » so sanh, bo sach, co sach
xanh di » vanh dai, xanh tai
danh di » danh du, danh dien, danh gia
thuc va » thuc vat, thuc dan, thuc day
van cac » van hac, van cau, van bac
nang ma » tang ma, sang ma, nang cao
nang xa » nang x, lang xa, mang xa
cua tre » cua te, cuu tre, cua triet

You may be able to get more results by adjusting your search query.

• Performing a fuzzy search may retrieve terms with similar spellings: so sach y ((nang xa) or (nang ma)) hoi. hieu van cac thuc va ((danh di) OR (xanh di)) cua tre~.
• If you are trying to use Boolean operators, they must be ALL CAPS: so sach y ((nang xa) OR (nang ma)) hoi. hieu van cac thuc va ((danh di) OR (xanh di)) cua tre.
• The words AND, OR and NOT may confuse the search; try adding quotes: "so sach y ((nang xa) or (nang ma)) hoi. hieu van cac thuc va ((danh di) OR (xanh di)) cua tre".
• Adding a wildcard symbol may retrieve word variants: so sach y ((nang xa) or (nang ma)) hoi. hieu van cac thuc va ((danh di) OR (xanh di)) cua tre*.