Perhaps you should try some spelling variations:
dua tren » dung tren
dat tren » dat trong
quoc ban » quoc bao, quoc lan, quoc quan
cam quoc » cac quoc, cao quoc, nam quoc
ban anh » ban hanh, ban banh, ban danh
sach ky » sach y, sach ty, sach kem
hieu va » lieu va, hieu qua
nang ro » nang co, nang ho, nang ra
ro hoc » co hoc, mo hoc, o hoc
sac ky » cac ky, sai ky
va cam » va cham, va cac, va cao

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

• Performing a fuzzy search may retrieve terms with similar spellings: tu ((sach ky) or (sac ky)) nang ro hoc. hieu va cam quoc ban anh d ((dat tren) OR (dua tren))~.
• If you are trying to use Boolean operators, they must be ALL CAPS: tu ((sach ky) OR (sac ky)) nang ro hoc. hieu va cam quoc ban anh d ((dat tren) OR (dua tren)).
• The words AND, OR and NOT may confuse the search; try adding quotes: "tu ((sach ky) or (sac ky)) nang ro hoc. hieu va cam quoc ban anh d ((dat tren) OR (dua tren))".
• Adding a wildcard symbol may retrieve word variants: tu ((sach ky) or (sac ky)) nang ro hoc. hieu va cam quoc ban anh d ((dat tren) OR (dua tren))*.