Perhaps you should try some spelling variations:
hieu van » hieu va, thien van, lieu va
cau tren » cu tren, dau tren, cau treo
cau thuc » cau truc, cuu thuc, can thuc
su sach » vu sach
dan hai » doan hai, dang hai, dan khai
thuc va » thuc vat, thuc dan, thuc day
van cau » van dau, van hau, van chat
nang ma » tang ma, sang ma, nang cao
so sach » so sanh, bo sach, co sach
ca tren » cu tren, la tren, co tren
ma hoi » mai hoi, xa hoi, ma hoa
va dan » ca dan, va dao, va dau
y nang » ky nang, ly nang, v nang

You may be able to get more results by adjusting your search query.

• Performing a fuzzy search may retrieve terms with similar spellings: ((su sach) or (so sach)) y nang ma hoi. hieu van cau thuc va dan hai ((cau tren) OR (ca tren))~.
• If you are trying to use Boolean operators, they must be ALL CAPS: ((su sach) OR (so sach)) y nang ma hoi. hieu van cau thuc va dan hai ((cau tren) OR (ca tren)).
• The words AND, OR and NOT may confuse the search; try adding quotes: "((su sach) or (so sach)) y nang ma hoi. hieu van cau thuc va dan hai ((cau tren) OR (ca tren))".
• Adding a wildcard symbol may retrieve word variants: ((su sach) or (so sach)) y nang ma hoi. hieu van cau thuc va dan hai ((cau tren) OR (ca tren))*.