Perhaps you should try some spelling variations:
hieu quang » thieu quang, hieu quan, hieu qua
thong cong » trong cong, tong cong, thong thong
hieu lang » chieu sang, nhieu tang, hieu van
hanh tang » hanh trang, anh tang, hanh mang
dang cong » dung cong
dan toan » van toan, dan than, manh toan
toan cao » toan ban, toan tap
tang va » trang va, tang van, thang va

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

• If you are trying to use Boolean operators, they must be ALL CAPS: ho ((hieu lang) OR (hieu quang)) - hanh tang va ((thong cong) OR (dang cong)) dan toan cao~.
• The words AND, OR and NOT may confuse the search; try adding quotes: "ho ((hieu lang) OR (hieu quang)) - hanh tang va ((thong cong) or (dang cong)) dan toan cao~".
• Adding a wildcard symbol may retrieve word variants: ho ((hieu lang) OR (hieu quang)) - hanh tang va ((thong cong) or (dang cong)) dan toan cao*.