Perhaps you should try some spelling variations:
toc sach » hoc sach, doc sach, thuoc sach
cac nuoc » cap nuoc, hoc nuoc, dat nuoc
ky sang » ky dang, ky tang, duy sang
hieu ve » hieu va, hieu van
ma hoi » mai hoi, xa hoi, ma hoa
ve cac » va cac, vien cac
va dan » ca dan, va dao, va dau
dan vi » doan vi, don vi, dan va
vi cau » va cau, ve cau, di cau

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) or (ky nang)) ma hoi. hieu ve cac nuoc va dan vi cau tao~.
• If you are trying to use Boolean operators, they must be ALL CAPS: toc sach ((ky sang) OR (ky nang)) ma hoi. hieu ve cac nuoc va dan vi cau tao.
• Adding a wildcard symbol may retrieve word variants: toc sach ((ky sang) or (ky nang)) ma hoi. hieu ve cac nuoc va dan vi cau tao*.