Perhaps you should try some spelling variations:
"doc canh tranh tren" » "doc canh trinh tren", "doc canh tran tren", "doc canh tranh trong", "doc hanh tranh tren", "do canh tranh tren", "doi canh tranh tren"
"tu sach dinh tren" » "tu sach kinh tren", "tu sach danh tren", "tu hoach dinh tren", "tu sach dinh trong", "tu sach dinh tri", "tu sach dinh dien", "thu sach dinh tren", "su sach dinh tren", "te sach dinh tren"
"toc sach dong tren" » "toc sach dung tren", "toc sach song tren", "toc sach rong tren", "toc sach dong tien", "toc sach dong truyen", "hoc sach dong tren", "doc sach dong tren", "thuoc sach dong tren"

Try widening your search to All Fields.

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

• Performing a fuzzy search may retrieve terms with similar spellings: (("tu sach dinh tren") or ((("toc sach dong tren") or ("doc canh tranh tren"))))~.
• If you are trying to use Boolean operators, they must be ALL CAPS: (("tu sach dinh tren") OR ((("toc sach dong tren") OR ("doc canh tranh tren")))).
• Removing quotes may allow a broader search: (( tu sach dinh tren ) or ((( toc sach dong tren ) or ( doc canh tranh tren )))).
• Adding a wildcard symbol may retrieve word variants: (("tu sach dinh tren") or ((("toc sach dong tren") or ("doc canh tranh tren"))))*.