Perhaps you should try some spelling variations:
nhung cong » dung cong, thong cong, hang cong
hieu lang » chieu sang, hieu quang, nhieu tang
tang van » dang van, hoang van, trong van
tap toan » tap doan, tap quan
bai toan » bai tap
ho hieu » ho chieu, hoi hieu, nho hieu
tang va » trang va, thang va, tang vat

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