Perhaps you should try some spelling variations:
toc sach » hoc sach, doc sach, thuoc sach
thuc may » thuc day, truc may, phuc may
cac thuc » cach thuc, cac thu, can thuc
su sach » so sach, vu sach
tinh va » tich va, trinh van, trinh vat
hieu ve » hieu va, hieu van
van hoc » van hoa
van hoi » van hoa, van hoe, van loi
ky tang » ky nang, ky dang, kho tang
cua tre » cua te, cuu tre, cua triet
ve cac » va cac, vien cac
va cua » va chua, via cua, ca cua

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 (toc sach)) ky tang ((van hoi) OR (van hoc)). hieu ve cac thuc may tinh va cua tre~.
• If you are trying to use Boolean operators, they must be ALL CAPS: ((su sach) OR (toc sach)) ky tang ((van hoi) OR (van hoc)). hieu ve cac thuc may tinh va cua tre.
• The words AND, OR and NOT may confuse the search; try adding quotes: "((su sach) or (toc sach)) ky tang ((van hoi) OR (van hoc)). hieu ve cac thuc may tinh va cua tre".
• Adding a wildcard symbol may retrieve word variants: ((su sach) or (toc sach)) ky tang ((van hoi) OR (van hoc)). hieu ve cac thuc may tinh va cua tre*.