Perhaps you should try some spelling variations:
"so sach ky thuat co khi" » "so sach ky thuat do khi", "so sach ky thuat o khi", "bo sach ky thuat co khi", "co sach ky thuat co khi", "hoc sach ky thuat co khi", "so sac ky thuat co khi", "so sach y thuat co khi", "so sach ty thuat co khi"
"so sanh ky thuat co khi" » "so sanh ky thuat do khi", "so sanh ky thuat o khi", "do sanh ky thuat co khi", "so hanh ky thuat co khi", "so nganh ky thuat co khi", "so thanh ky thuat co khi"
"so sang ky thuat co khi" » "so sang ky thuat do khi", "so sang ky thuat o khi", "so dang ky thuat co khi", "so lang ky thuat co khi", "so bang ky thuat co khi", "so sang kyo thuat co khi", "so tang ky thuat co 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 ((("so sanh ky thuat co khi") OR ("so sang ky thuat co khi"))))~.
• If you are trying to use Boolean operators, they must be ALL CAPS: (("so sach ky thuat co khi") OR ((("so sanh ky thuat co khi") OR ("so sang ky thuat co khi")))).
• The words AND, OR and NOT may confuse the search; try adding quotes: "((\"so sach ky thuat co khi\") or (((\"so sanh ky thuat co khi\") OR (\"so sang ky thuat co khi\"))))".
• Removing quotes may allow a broader search: (( so sach ky thuat co khi ) or ((( so sanh ky thuat co khi ) OR ( so sang ky thuat co khi )))).
• Adding a wildcard symbol may retrieve word variants: (("so sach ky thuat co khi") or ((("so sanh ky thuat co khi") OR ("so sang ky thuat co khi"))))*.