Perhaps you should try some spelling variations:
khau thong » khau thuong, khap thong, khau tong
giai khoa » giao khoa, ngoai khoa, giai thoai
pho thong » he thong
sach bai » sach dao, cac bai, sach xa
tac giai » tac gia, cac giai, tap giai
su sach » so sach, vu sach
bai tac » bai tap, bai toan, bai tang

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

• Performing a fuzzy search may retrieve terms with similar spellings: su sach bai tac giai khoa ((khau thong) or (pho thong))~.
• If you are trying to use Boolean operators, they must be ALL CAPS: su sach bai tac giai khoa ((khau thong) OR (pho thong)).
• Adding a wildcard symbol may retrieve word variants: su sach bai tac giai khoa ((khau thong) or (pho thong))*.