sudo apt-get update not working due to proxy changes in ubuntu 12.04

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http://askubuntu.com – My university set proxy to use Internet for us. I set proxy "" for using internet in university. Now when in home I run "sudo apt-get update" it generates an error although I set default options of proxies in Internet options. Err http://pk.archive.ubuntu.com precise Release.gpg Unable to connect to Err http://pk.archive.ubuntu.com precise-updates Release.gpg Unable to connect to E: Some index files failed to download. They have been ignored, or old ones used instead. (HowTos)