Perhaps you should try some spelling variations:
hoc sach » hoc sinh, hoc cac, hoc anh
cau thuc » cau truc, cuu thuc, can thuc
mai hoi » mai khoi, dai hoi, lai hoi
hieu ve » hieu va, hieu van
sach ky » sac ky, sach y, sach ty
thuc va » thuc vat, thuc dan, thuc day
va danh » va dinh, va dan, va hanh
vi dau » vi dai, va dau, ve dau
ve cau » ke cau, va cau, ve cac

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

• Performing a fuzzy search may retrieve terms with similar spellings: hoc sach ky nang ((xa hoi) or (mai hoi)). hieu ve cau thuc va danh ((vi dau) OR (bi dau)) tren~.
• If you are trying to use Boolean operators, they must be ALL CAPS: hoc sach ky nang ((xa hoi) OR (mai hoi)). hieu ve cau thuc va danh ((vi dau) OR (bi dau)) tren.
• The words AND, OR and NOT may confuse the search; try adding quotes: "hoc sach ky nang ((xa hoi) or (mai hoi)). hieu ve cau thuc va danh ((vi dau) OR (bi dau)) tren".
• Adding a wildcard symbol may retrieve word variants: hoc sach ky nang ((xa hoi) or (mai hoi)). hieu ve cau thuc va danh ((vi dau) OR (bi dau)) tren*.