Perhaps you should try some spelling variations:
trung tuan » trung thanh, trung tam, tran tuan
nhung cong » dung cong, thong cong, hang cong
hieu lang » chieu sang, hieu quang, nhieu tang
trung cua » truong cua, trung cap, dung cua
tap toan » tap doan, tap quan
bai toan » bai tap
ho hieu » ho chieu, hoi hieu, nho hieu

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