Perhaps you should try some spelling variations:
nang cau » nang cao, hang cau, nang dau
hieu van » hieu va, thien van, lieu va
cam thuc » nam thuc, cam thu, can thuc
cua viet » ca viet, hoa viet, cua vat
sach ky » sach y, sach ty, sach kem
va hanh » van hanh, va thanh, va danh
sac ky » cac ky, sai ky
vi cua » vai cua, di cua

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

• Performing a fuzzy search may retrieve terms with similar spellings: so ((sach ky) or (sac ky)) nang cau hoi. hieu van ((cam thuc) OR (am thuc)) va hanh vi cua viet~.
• If you are trying to use Boolean operators, they must be ALL CAPS: so ((sach ky) OR (sac ky)) nang cau hoi. hieu van ((cam thuc) OR (am thuc)) va hanh vi cua viet.
• The words AND, OR and NOT may confuse the search; try adding quotes: "so ((sach ky) or (sac ky)) nang cau hoi. hieu van ((cam thuc) OR (am thuc)) va hanh vi cua viet".
• Adding a wildcard symbol may retrieve word variants: so ((sach ky) or (sac ky)) nang cau hoi. hieu van ((cam thuc) OR (am thuc)) va hanh vi cua viet*.