How to check which packages are installed? to remove them?

[Antonio Olivares <olivares14031 yahoo com>]

Dear List,
   On one of my computers, System Settings ->
Add/Remove Applications is not working as it should. 
So I removed it within itself, now I cannot see which
packages are installed.  I know that the command(s)
have been discussed before but I probably deleted the
messages. rpm -??.  Is there a way via command line to
determine which packages are installed in the system
and how to remove them?  

Also I would like to ask on how to remove
documentation from the system.  I need to know how
much space help/man pages occupies.  I need to free up
some space and want to know how to do it.

Regards,

Antonio

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