Perhaps you should try some spelling variations:
"so sach ky thuat co khi" » "so sach ky thuat o khi", "so sanh ky thuat co khi", "bo sach ky thuat co khi", "co sach ky thuat co khi", "so sac ky thuat co khi", "so sach y thuat co khi", "so sach ty thuat co khi"
"su sai ky thuat do khi" » "su sai ky thuat o khi", "su tai ky thuat do khi", "su dai ky thuat do khi", "su mai ky thuat do khi", "su sac ky thuat do khi", "su sai ky thuat doi khi", "su sai ky thuat do thi", "su sai ky thuat bo khi"
"su hai ky thuat do khi" » "su hai ky thuat o khi", "su hai ky thuat doi khi", "su hai ky thuat do thi", "su hai ky thuat bo khi", "su tai ky thuat do khi", "su thai ky thuat do khi", "su hoi ky thuat do khi", "su hai ly thuat do khi"

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: (("so sach ky thuat co khi") or ((("su sai ky thuat do khi") OR ("su hai ky thuat do khi"))))~.
• If you are trying to use Boolean operators, they must be ALL CAPS: (("so sach ky thuat co khi") OR ((("su sai ky thuat do khi") OR ("su hai ky thuat do khi")))).
• The words AND, OR and NOT may confuse the search; try adding quotes: "((\"so sach ky thuat co khi\") or (((\"su sai ky thuat do khi\") OR (\"su hai ky thuat do khi\"))))".
• Removing quotes may allow a broader search: (( so sach ky thuat co khi ) or ((( su sai ky thuat do khi ) OR ( su hai ky thuat do khi )))).
• Adding a wildcard symbol may retrieve word variants: (("so sach ky thuat co khi") or ((("su sai ky thuat do khi") OR ("su hai ky thuat do khi"))))*.