Perhaps you should try some spelling variations:
tang mai » tang ma, tang tai, lang mai
hieu van » hieu va, thien van, lieu va
co sach » bo sach, cuon sach, hoc sach
mai hoi » mai khoi, dai hoi, lai hoi
van cac » van hac, van cau, van bac
dac xa » tac xa, dan xa, dac xuan
cac xa » tac xa, cac ba, cac xe
vi va » vi van, vai va, vu va
thiet » thieu

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

• Performing a fuzzy search may retrieve terms with similar spellings: ((do sanh) or (co sach)) 6 tang mai hoi. hieu van cac ((dac xa) OR (cac xa)) hanh vi va thiet~.
• If you are trying to use Boolean operators, they must be ALL CAPS: ((do sanh) OR (co sach)) 6 tang mai hoi. hieu van cac ((dac xa) OR (cac xa)) hanh vi va thiet.
• The words AND, OR and NOT may confuse the search; try adding quotes: "((do sanh) or (co sach)) 6 tang mai hoi. hieu van cac ((dac xa) OR (cac xa)) hanh vi va thiet".
• Adding a wildcard symbol may retrieve word variants: ((do sanh) or (co sach)) 6 tang mai hoi. hieu van cac ((dac xa) OR (cac xa)) hanh vi va thiet*.