Perhaps you should try some spelling variations:
danh cong » dang cong, hanh cong, dinh cong
anh cong » hanh cong, anh dong, thanh cong
hieu va » lieu va, hieu qua
cac duc » cac du, cao duc, cac dac
tu sach » thu sach, su sach, te sach
sach ky » sac ky, sach y, sach ty
nang ma » tang ma, sang ma, nang cao
mang xa » lang xa, mang may, dang xay
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 ((mang xa) or (nang ma)) hoi. hieu va cac duc va ((danh cong) OR (anh cong)) cua tre~.
• If you are trying to use Boolean operators, they must be ALL CAPS: tu sach ky ((mang xa) OR (nang ma)) hoi. hieu va cac duc va ((danh cong) OR (anh cong)) cua tre.
• The words AND, OR and NOT may confuse the search; try adding quotes: "tu sach ky ((mang xa) or (nang ma)) hoi. hieu va cac duc va ((danh cong) OR (anh cong)) cua tre".
• Adding a wildcard symbol may retrieve word variants: tu sach ky ((mang xa) or (nang ma)) hoi. hieu va cac duc va ((danh cong) OR (anh cong)) cua tre*.