Perhaps you should try some spelling variations:
hanh doi » thanh doi, hanh noi, hang doi
van hanh » van thanh, ban hanh, van anh
cam thuc » nam thuc, cam thu, can thuc
so sach » so sanh, bo sach, co sach
doi cua » dai cua, hoi cua
hieu ve » hieu va, hieu van
cua tre » cua te, cuu tre, cua triet
mang xa » lang xa, mang may, dang xay
sach v » sach va, sach y, sach 2

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 v ((mang xa) or (nang x)) hoi. hieu ve ((cam thuc) OR (am thuc)) van hanh doi cua tre~.
• If you are trying to use Boolean operators, they must be ALL CAPS: so sach v ((mang xa) OR (nang x)) hoi. hieu ve ((cam thuc) OR (am thuc)) van hanh doi cua tre.
• The words AND, OR and NOT may confuse the search; try adding quotes: "so sach v ((mang xa) or (nang x)) hoi. hieu ve ((cam thuc) OR (am thuc)) van hanh doi cua tre".
• Adding a wildcard symbol may retrieve word variants: so sach v ((mang xa) or (nang x)) hoi. hieu ve ((cam thuc) OR (am thuc)) van hanh doi cua tre*.