Perhaps you should try some spelling variations:
toc sach » hoc sach, thuoc sach, nuoc sach
nang mai » nang ma, nang tai, lang mai
su triet » su viet, su tien, ve triet
doc sach » hoc sach, nuoc sach, dac sac
mai hoi » mai khoi, dai hoi, lai hoi
hieu va » lieu va, hieu qua
duoc va » nuoc va, duoc ma, duong va
v nang » va nang, vu nang, van nang
dan vi » doan vi, don vi, dan va
vi su » va su, ve su, ki su

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) OR (doc sach)) v nang mai hoi. hieu va che ((duoc va) or (duc va)) dan vi su triet~.
• If you are trying to use Boolean operators, they must be ALL CAPS: ((toc sach) OR (doc sach)) v nang mai hoi. hieu va che ((duoc va) OR (duc va)) dan vi su triet.
• The words AND, OR and NOT may confuse the search; try adding quotes: "((toc sach) OR (doc sach)) v nang mai hoi. hieu va che ((duoc va) or (duc va)) dan vi su triet".
• Adding a wildcard symbol may retrieve word variants: ((toc sach) OR (doc sach)) v nang mai hoi. hieu va che ((duoc va) or (duc va)) dan vi su triet*.