Perhaps you should try some spelling variations:
"dong tac san van." » "dong tai san van." (Expand Search), "dong bac san van." (Expand Search), "dong tay san van." (Expand Search), "dong thac san van." (Expand Search), "dong tac van van." (Expand Search), "dong tac ban van." (Expand Search), "dong tac luan van." (Expand Search)
"cong tac sang van." » "cong tac dang van." (Expand Search), "cong tac vang van." (Expand Search), "cong cac sang van." (Expand Search), "cong tac soang van." (Expand Search), "cong tac song van." (Expand Search), "cong thac sang van." (Expand Search), "cong tac bang van." (Expand Search)
"cong tac san van." » "cong thac san van." (Expand Search), "cong tac van van." (Expand Search), "cong tac ban van." (Expand Search), "cong tac luan van." (Expand Search)

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• Performing a fuzzy search may retrieve terms with similar spellings: (((("cong tac san van.") OR ("dong tac san van."))) OR ("cong tac sang van."))~.
• The words AND, OR and NOT may confuse the search; try adding quotes: "((((\"cong tac san van.\") OR (\"dong tac san van.\"))) OR (\"cong tac sang van.\"))".
• Removing quotes may allow a broader search: (((( cong tac san van. ) OR ( dong tac san van. ))) OR ( cong tac sang van. )).
• Adding a wildcard symbol may retrieve word variants: (((("cong tac san van.") OR ("dong tac san van."))) OR ("cong tac sang van."))*.