Perhaps you should try some spelling variations:
nam thuc » nam thuoc, am thuc, nam thu
dang xay » dang van, van xay
hieu cua » hieu qua, dieu cua, bieu cua
su sach » so sach, vu sach
doan vi » doan va, don vi, doan thi
thuc va » thuc vat, thuc dan, thuc day
sach ty » sach y, sach tu, sach thu
cua nam » cua tam, cua ngan, cua nhan
lang xa » lang ha, lang van
cua tre » cua te, cuu tre, cua triet
dan vi » don vi, dan va, gan vi

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

• Performing a fuzzy search may retrieve terms with similar spellings: su sach ty ((lang xa) or (dang xay)) hoi. hieu cua nam thuc va ((dan vi) OR (doan vi)) cua tre~.
• If you are trying to use Boolean operators, they must be ALL CAPS: su sach ty ((lang xa) OR (dang xay)) hoi. hieu cua nam thuc va ((dan vi) OR (doan vi)) cua tre.
• The words AND, OR and NOT may confuse the search; try adding quotes: "su sach ty ((lang xa) or (dang xay)) hoi. hieu cua nam thuc va ((dan vi) OR (doan vi)) cua tre".
• Adding a wildcard symbol may retrieve word variants: su sach ty ((lang xa) or (dang xay)) hoi. hieu cua nam thuc va ((dan vi) OR (doan vi)) cua tre*.