Perhaps you should try some spelling variations:
"cong thac san van." » "cong thuc san van." (Expand Search), "cong thao san van." (Expand Search), "cong than san van." (Expand Search), "cong nhac san van." (Expand Search), "cong thac ban van." (Expand Search), "cong thac luan van." (Expand Search)
"dong tac dan van." » "dong tai dan van." (Expand Search), "dong bac dan van." (Expand Search), "dong tay dan van." (Expand Search), "dong cac dan van." (Expand Search), "dong tac van van." (Expand Search), "dong sac dan van." (Expand Search), "dong tac dang van." (Expand Search), "dong tac doan van." (Expand Search), "dong tac tan van." (Expand Search)
"cong tac san van." » "cong tac sang 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: (("dong tac dan van.") OR ((("cong tac san van.") OR ("cong thac san van."))))~.
• The words AND, OR and NOT may confuse the search; try adding quotes: "((\"dong tac dan van.\") OR (((\"cong tac san van.\") OR (\"cong thac san van.\"))))".
• Removing quotes may allow a broader search: (( dong tac dan van. ) OR ((( cong tac san van. ) OR ( cong thac san van. )))).
• Adding a wildcard symbol may retrieve word variants: (("dong tac dan van.") OR ((("cong tac san van.") OR ("cong thac san van."))))*.