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 hai ky thuat do khi", "su dai ky thuat do khi", "su sac ky thuat do khi", "su sai ky thuat doi khi", "su sai ky thuat bo khi", "su sai ky thuat dong khi"
"su sai ky thuat do thi" » "su sai ky thuat o thi", "su tai ky thuat do thi", "su hai ky thuat do thi", "su dai ky thuat do thi", "su sac ky thuat do thi"

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: (("so sach ky thuat co khi") OR ((("su sai ky thuat do khi") OR ("su sai ky thuat do thi"))))~.
• 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 sai ky thuat do thi\"))))~".
• Removing quotes may allow a broader search: (( so sach ky thuat co khi ) or ((( su sai ky thuat do khi ) OR ( su sai ky thuat do thi ))))~.
• Adding a wildcard symbol may retrieve word variants: (("so sach ky thuat co khi") or ((("su sai ky thuat do khi") OR ("su sai ky thuat do thi"))))*.