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Thank you for contacting Intel Communities.
In this case, the compatibility information should be provided by the system manufacturer (Dell), as there are some other specifications that you may take a look other than socket size, such as BIOS compatibility, thermal design, etc.
Since you mentioned that you tried contacting Dell but you were unable to get the information you were looking for, I did some further research and found the following that you may find useful:
Optiplex 755 Processor Upgrade - Dell Community.
Please also check this article regarding processor upgrade information.
I hope this information helps.