Perhaps you should try some spelling variations:
khoan phai » khong phai, khoa phau, khoan hoi
phai tuong » phan thuong, pham thuong, pham truong
pho thong » he thong
bai tich » giai tich, bi tinh, di tich
tich cac » tich va, tich bao
sach tai » sach bai, sach tham, sach dao
tai thac » tai thanh, khai thac, mai thach
thac cac » thoai cac, thanh cac, thuc cach
cac khoa » cac khoi, dac khoa, cao khoa
khoa pho » khoa phu, khoa khoa, khoa phau
tu sach » thu sach, su sach, te sach

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

• Performing a fuzzy search may retrieve terms with similar spellings: (( tu cac bai tich cac khoan phai tuong ) or ( tu sach tai thac cac khoa pho thong ))~.
• If you are trying to use Boolean operators, they must be ALL CAPS: (( tu cac bai tich cac khoan phai tuong ) OR ( tu sach tai thac cac khoa pho thong )).
• Adding a wildcard symbol may retrieve word variants: (( tu cac bai tich cac khoan phai tuong ) or ( tu sach tai thac cac khoa pho thong ))*.