Perhaps you should try some spelling variations:
dan thanh » dang thanh, danh thanh, ban thanh
toc sach » hoc sach, doc sach, thuoc sach
thuc dan » thuc hanh, hoc dan, thuc vat
cam thuc » am thuc, nam thuc, cam thu
vai cua » hai cua, lai cua, bai cua
hieu ve » hieu va, hieu van
tay hoi » tay ho, tao hoi, tay thai
xay hoi » xa hoi, cay phoi, gay thoi
sach o » sach lo, sac o, sach y
ve cam » ve cham, ve cac, va cam
o dang » xo dang, do dang, so dang

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 o dang ((xay hoi) OR (tay hoi)). hieu ve cam thuc dan thanh ((vai cua) or (dai cua)) tre~.
• If you are trying to use Boolean operators, they must be ALL CAPS: toc sach o dang ((xay hoi) OR (tay hoi)). hieu ve cam thuc dan thanh ((vai cua) OR (dai cua)) tre.
• The words AND, OR and NOT may confuse the search; try adding quotes: "toc sach o dang ((xay hoi) OR (tay hoi)). hieu ve cam thuc dan thanh ((vai cua) or (dai cua)) tre".
• Adding a wildcard symbol may retrieve word variants: toc sach o dang ((xay hoi) OR (tay hoi)). hieu ve cam thuc dan thanh ((vai cua) or (dai cua)) tre*.