Perhaps you should try some spelling variations:
trinh toan » tinh toan, thanh toan, trinh han
trinh van » trinh han, trinh quan, dinh van
hieu van » hieu va, thien van, lieu va
nam thuc » nam thuoc, am thuc, nam thu
thuc dan » thuc hanh, hoc dan, thuc vat
tu sach » thu sach, su sach, te sach
sach ty » sach y, sach tu, sach thu
nang xa » lang xa, nang ma, 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: tu sach ty ((nang xa) or (nang x)) hoi. hieu van nam thuc dan ((trinh van) OR (trinh toan)) cua tre~.
• If you are trying to use Boolean operators, they must be ALL CAPS: tu sach ty ((nang xa) OR (nang x)) hoi. hieu van nam thuc dan ((trinh van) OR (trinh toan)) cua tre.
• The words AND, OR and NOT may confuse the search; try adding quotes: "tu sach ty ((nang xa) or (nang x)) hoi. hieu van nam thuc dan ((trinh van) OR (trinh toan)) cua tre".
• Adding a wildcard symbol may retrieve word variants: tu sach ty ((nang xa) or (nang x)) hoi. hieu van nam thuc dan ((trinh van) OR (trinh toan)) cua tre*.