Perhaps you should try some spelling variations:
van nang » van quang, van ban, van dong
van hanh » van thanh, ban hanh, van anh
truc van » truc va, thuc van, truc ban
sach van » sach han, sach va, bach van
so sach » so sanh, bo sach, co sach
cuu tre » cau tren, cau treo
hanh de » hanh ve, hanh kem
hieu ve » hieu va, hieu van
cua tre » cua te, cua triet, dua tren
ve cau » ke cau, va cau, ve cac
o hoc » co hoc, mo hoc, so hoc
o hoi » co hoi, o hai, o hoa

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 van nang ((o hoi) or (o hoc)). hieu ve cau truc van hanh de ((cuu tre) OR (cua tre))~.
• If you are trying to use Boolean operators, they must be ALL CAPS: so sach van nang ((o hoi) OR (o hoc)). hieu ve cau truc van hanh de ((cuu tre) OR (cua tre)).
• The words AND, OR and NOT may confuse the search; try adding quotes: "so sach van nang ((o hoi) or (o hoc)). hieu ve cau truc van hanh de ((cuu tre) OR (cua tre))".
• Adding a wildcard symbol may retrieve word variants: so sach van nang ((o hoi) or (o hoc)). hieu ve cau truc van hanh de ((cuu tre) OR (cua tre))*.