Perhaps you should try some spelling variations:
toc sach » hoc sach, doc sach, thuoc sach
cam thuc » am thuc, nam thuc, cam thu
hanh tai » thanh tai, hang tai, han tai
van hanh » van thanh, ban hanh, van anh
hieu van » thien van, lieu va, hieu ve
thuc van » thuc vat, thuc dan, thuc va
nang xa » nang x, lang xa, mang xa
tai cua » thai cua, tai cuc, tri cua
nang ma » tang ma, sang ma, nang cao
sach ky » sac ky, sach y, sach ty
hieu va » lieu va, hieu qua
cua te » cua tu

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 ((nang xa) or (nang ma)) hoi. ((hieu van) OR (hieu va)) cam thuc van hanh tai cua te~.
• If you are trying to use Boolean operators, they must be ALL CAPS: toc sach ky ((nang xa) OR (nang ma)) hoi. ((hieu van) OR (hieu va)) cam thuc van hanh tai cua te.
• The words AND, OR and NOT may confuse the search; try adding quotes: "toc sach ky ((nang xa) or (nang ma)) hoi. ((hieu van) OR (hieu va)) cam thuc van hanh tai cua te".
• Adding a wildcard symbol may retrieve word variants: toc sach ky ((nang xa) or (nang ma)) hoi. ((hieu van) OR (hieu va)) cam thuc van hanh tai cua te*.