So you are asking what package contains the file that has sqrt.3 as part of its filename.
If you have apt-file search installed and set up, you could use that for such questions.
If you want to do such searches online, the Ubuntu answer is easy to get from
http://packages.ubuntu.com/
and the Ubuntu answer regarding most package names is the same as the package name in other Debian based distributions.
For sqrt.3, the Ubuntu answer is at
http://packages.ubuntu.com/search?se...lucid&arch=any
In that answer, if you ignore the language and otherwise over specialized choices, you can spot the general choice, which is the package
manpages-dev
I assume you know how to install that package.