sudo apt get/full-screen in virtual box

1>open /etc/apt from terminal
2>create apt.conf (if not present) and add
ACQUIRE{
http::proxy "http://username:passwd@proxy-name:port_number/"
}
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1>VBOXAddition_3(cd folder) (where we get VBoxLinux Additions_x86.run)
2>copy VBoxLinux Additions_x86.run to desktop and then  run
sudo sh VBoxLinuxAdditions_x86.run