Perhaps you should try some spelling variations:
duoc tren » nuoc tren, thuoc tren, dung tren
van hanh » van thanh, ban hanh, van anh
dang mam » dang nam, dang mau, dang mat
hanh tai » thanh tai, hang tai, han tai
cham luc » cham soc, nhan luc, cham cuu
tu sach » thu sach, su sach, te sach
duc tre » duc tien
luc van » luoc van, luu van, duc van
sach ky » sac ky, sach y, sach ty
va cham » va chat, va tham
hieu va » lieu va, hieu qua
mam hoi » mam hoai, nam hoi, mai hoi
ky dang » ky nang, ky danh
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 ky dang mam hoi. hieu va cham luc van hanh tai ((((duc tre) or (duoc tren))) or (cua tre))~.
• If you are trying to use Boolean operators, they must be ALL CAPS: tu sach ky dang mam hoi. hieu va cham luc van hanh tai ((((duc tre) OR (duoc tren))) OR (cua tre)).
• Adding a wildcard symbol may retrieve word variants: tu sach ky dang mam hoi. hieu va cham luc van hanh tai ((((duc tre) or (duoc tren))) or (cua tre))*.