Perhaps you should try some spelling variations:
dung gian » duong gian, dung giao, dung giam
hieu soat » hieu suat, hieu hoat, hieu luat
quang tan » quang tuan, quang tran, quang toan
gian dan » gian lang
dan hieu » danh hieu, van hieu, dao hieu
van hien » van hieu
van tien » toan tien, van hieu, van tuan

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: (( trinh dung gian dan hieu soat ) OR ( tinh quang tan ((van hien) OR (van tien)) soat ))~.
• Adding a wildcard symbol may retrieve word variants: (( trinh dung gian dan hieu soat ) or ( tinh quang tan ((van hien) or (van tien)) soat ))*.