Perhaps you should try some spelling variations:
toc sach » hoc sach, doc sach, thuoc sach
sang mai » sang ma, sang tai, lang mai
cac nuoc » cap nuoc, hoc nuoc, dat nuoc
cau theo » cau thi, cau tao
sach ky » sac ky, sach y, sach ty
ky sang » ky nang, ky dang, ky tang
mai hoi » mai khoi, dai hoi, lai hoi
hieu ve » hieu va, hieu van
doan vi » doan va, don vi, doan thi
ve cac » va cac, vien cac
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: toc sach ky sang mai hoi. hieu ve cac nuoc va ((dan vi) or (doan vi)) cau theo~.
• If you are trying to use Boolean operators, they must be ALL CAPS: toc sach ky sang mai hoi. hieu ve cac nuoc va ((dan vi) OR (doan vi)) cau theo.
• Adding a wildcard symbol may retrieve word variants: toc sach ky sang mai hoi. hieu ve cac nuoc va ((dan vi) or (doan vi)) cau theo*.