Perhaps you should try some spelling variations:
than soc » than hoc, than so, thanh son
ban hang » ngan hang
ky tang » ky nang, ky dang, kho tang
mai hoi » mai khoi, lai hoi, mai doi
dai hoi » dai hoc, dai hai, dai moi
cua tre » cua te, cuu tre, cua triet
va cua » va chua, via cua, ca cua

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

• If you are trying to use Boolean operators, they must be ALL CAPS: tu cac ky tang ((mai hoi) OR (dai hoi)). hieu qua ((than soc) OR (than doc)) ban hang va cua tre~.
• The words AND, OR and NOT may confuse the search; try adding quotes: "tu cac ky tang ((mai hoi) OR (dai hoi)). hieu qua ((than soc) or (than doc)) ban hang va cua tre~".
• Adding a wildcard symbol may retrieve word variants: tu cac ky tang ((mai hoi) OR (dai hoi)). hieu qua ((than soc) or (than doc)) ban hang va cua tre*.