Perhaps you should try some spelling variations:
hieu quang » thieu quang, hieu quan, hieu qua
hieu lang » chieu sang, nhieu tang, hieu van
hanh hang » thanh hang, hang hang, hanh han
dung cong » dung trong
tap toan » tap doan, tap quan, bai toan
van toan » van tuan, van tan, van thanh
va dung » van dung, va duong, vat dung

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 hang va dung cong ((tap toan) OR (van toan)) cao~.
• The words AND, OR and NOT may confuse the search; try adding quotes: "ho ((hieu lang) OR (hieu quang)) - hanh hang va dung cong ((tap toan) or (van toan)) cao~".
• Adding a wildcard symbol may retrieve word variants: ho ((hieu lang) OR (hieu quang)) - hanh hang va dung cong ((tap toan) or (van toan)) cao*.