Perhaps you should try some spelling variations:
khai » khac, hai, thai
pha" » phan", pham", phap"
phat" » phant", phamt", phapt", phan", pham", phap"
phanm" » phannm", phamnm", phapnm", phan", pham", phang"
phatm" » phantm", phamtm", phaptm", phanm", phamm", phapm", pham"
phatp" » phantp", phamtp", phaptp", phanp", phamp", phapp", phap"

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: "((khoa hoc) or (hoa hoc)) & khai (((pha" OR phat") or (phanm" OR phatm")) or phatp")~.
• Removing quotes may allow a broader search: ((khoa hoc) or (hoa hoc)) & khai (((pha or phat ) or (phanm or phatm )) or phatp )~.
• Adding a wildcard symbol may retrieve word variants: "((khoa hoc) or (hoa hoc)) & khai (((pha" or phat") or (phanm" or phatm")) or phatp")*.