Perhaps you should try some spelling variations:
doanh van » doan van, doanh va, doanh ban
hieu quan » lieu quan, thien quan, lien quan
danh vai » danh sai, danh va, dang van
tu sach » thu sach, su sach, te sach
cau duc » cao duc, cau luc, chu duc
sach so » mach so, sach lo, sach song
lang xa » lang ha, dang xay, lang van
so lang » o lang, so dang, hoa lang
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 so lang xa hoi. ((hieu quan) or (hieu qua)) cau duc va ((danh vai) OR (doanh van)) cua tre~.
• If you are trying to use Boolean operators, they must be ALL CAPS: tu sach so lang xa hoi. ((hieu quan) OR (hieu qua)) cau duc va ((danh vai) OR (doanh van)) cua tre.
• The words AND, OR and NOT may confuse the search; try adding quotes: "tu sach so lang xa hoi. ((hieu quan) or (hieu qua)) cau duc va ((danh vai) OR (doanh van)) cua tre".
• Adding a wildcard symbol may retrieve word variants: tu sach so lang xa hoi. ((hieu quan) or (hieu qua)) cau duc va ((danh vai) OR (doanh van)) cua tre*.