Perhaps you should try some spelling variations:
khanh toan » thanh toan, khanh thanh, khanh thuan
van khanh » van thanh, van hanh, van anh
cau tren » cu tren, dau tren, ca tren
hieu van » hieu va, thien van, lieu va
toan cau » toan cao, toan ban, toan tap
duc van » duc va, duc tan, duc manh
cac duc » cac du, cao duc, cac dac
tu sach » thu sach, su sach, te sach
sach ky » sac ky, sach y, sach ty
hieu ve » hieu va
co hoi » co hoc, co loi
o hoc » co hoc, mo hoc, so hoc

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 ky nang ((o hoc) or (co hoi)). ((hieu ve) OR (hieu van)) cac duc van khanh toan cau tren~.
• If you are trying to use Boolean operators, they must be ALL CAPS: tu sach ky nang ((o hoc) OR (co hoi)). ((hieu ve) OR (hieu van)) cac duc van khanh toan cau tren.
• The words AND, OR and NOT may confuse the search; try adding quotes: "tu sach ky nang ((o hoc) or (co hoi)). ((hieu ve) OR (hieu van)) cac duc van khanh toan cau tren".
• Adding a wildcard symbol may retrieve word variants: tu sach ky nang ((o hoc) or (co hoi)). ((hieu ve) OR (hieu van)) cac duc van khanh toan cau tren*.