Perhaps you should try some spelling variations:
cham truc » cau truc, nham thuc, pham thuc
cau tren » cu tren, dau tren, ca tren
doan vi » doan va, don vi, doan thi
truc va » thuc va, thuc vat, trung va
sach ky » sac ky, sach y, sach ty
hieu ve » hieu va, hieu van
tu sach » thu sach, su sach, te sach
nang ma » tang ma, nang cao, mang may
sang ma » tang ma, sang tao, mang may
ve cham » va cham, ve chat, vu cham
dan vi » don vi, dan va, gan vi

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 ((sang ma) or (nang ma)) hoi. hieu ve cham truc va ((dan vi) OR (doan vi)) cau tren~.
• If you are trying to use Boolean operators, they must be ALL CAPS: tu sach ky ((sang ma) OR (nang ma)) hoi. hieu ve cham truc va ((dan vi) OR (doan vi)) cau tren.
• The words AND, OR and NOT may confuse the search; try adding quotes: "tu sach ky ((sang ma) or (nang ma)) hoi. hieu ve cham truc va ((dan vi) OR (doan vi)) cau tren".
• Adding a wildcard symbol may retrieve word variants: tu sach ky ((sang ma) or (nang ma)) hoi. hieu ve cham truc va ((dan vi) OR (doan vi)) cau tren*.