Perhaps you should try some spelling variations:
cao tren » ca tren, cau tren, co tren
thai cao » tai cao, thoai cac, thuat cao
hieu van » hieu va, thien van, lieu va
cac toc » cac toi, cac nuoc, cac thoi
sach ky » sac ky, sach y, sach kem
lang xa » lang ha, dang xay, lang van
sach ty » sach y, sach tu, sach thu
van cac » van hac, van cau, van bac
va dan » ca dan, va dao, va dap
va dau » va dao

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) or (sach ty)) lang xa hoi. hieu van cac toc ((va dan) OR (va dau)) thai cao tren~.
• If you are trying to use Boolean operators, they must be ALL CAPS: tu ((sach ky) OR (sach ty)) lang xa hoi. hieu van cac toc ((va dan) OR (va dau)) thai cao tren.
• The words AND, OR and NOT may confuse the search; try adding quotes: "tu ((sach ky) or (sach ty)) lang xa hoi. hieu van cac toc ((va dan) OR (va dau)) thai cao tren".
• Adding a wildcard symbol may retrieve word variants: tu ((sach ky) or (sach ty)) lang xa hoi. hieu van cac toc ((va dan) OR (va dau)) thai cao tren*.