Perhaps you should try some spelling variations:
"co sach ky thuat o khi" » "bo sach ky thuat o khi", "cuon sach ky thuat o khi", "hoc sach ky thuat o khi", "co sac ky thuat o khi", "co sach y thuat o khi", "co sach ty thuat o khi", "co sach ky thuat bo khi"
"so sach ky thuat do khi" » "so sac ky thuat do khi", "so sach y thuat do khi", "so sach ty thuat do khi", "so sanh ky thuat do khi", "bo sach ky thuat do khi", "hoc sach ky thuat do khi", "so sach ky thuat doi khi", "so sach ky thuat do thi", "so sach ky thuat bo khi", "so sach ky thuat dbo khi"
"su sach ky thuat co khi" » "su sac ky thuat co khi", "su sach y thuat co khi", "su sach ty thuat co khi", "vu sach ky thuat co khi", "su sach ky thuat cbo khi"

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

• Performing a fuzzy search may retrieve terms with similar spellings: (((("co sach ky thuat o khi") or ("so sach ky thuat do khi"))) or ("su sach ky thuat co khi"))~.
• If you are trying to use Boolean operators, they must be ALL CAPS: (((("co sach ky thuat o khi") OR ("so sach ky thuat do khi"))) OR ("su sach ky thuat co khi")).
• Removing quotes may allow a broader search: (((( co sach ky thuat o khi ) or ( so sach ky thuat do khi ))) or ( su sach ky thuat co khi )).
• Adding a wildcard symbol may retrieve word variants: (((("co sach ky thuat o khi") or ("so sach ky thuat do khi"))) or ("su sach ky thuat co khi"))*.