Configuring proxy for APT in Ubuntu

Recently, I have a problem where when I tried to update Ubuntu package via apt-get, it shows HTTP 401 proxy error related.
Just a note, I’m running the VM using my office network which has a proxy servers.

From this site;

APT configuration file method
This method uses the apt.conf file which is found in your /etc/apt/ directory. This method is useful if you only want apt-get (and not other applications) to use a http-proxy permanently.
On some installations there will be no apt-conf file set up. This procedure will either edit an existing apt-conf file or create a new apt-conf file.
gksudo gedit /etc/apt/apt.conf
Add this line to your /etc/apt/apt.conf file (substitute your details for yourproxyaddress and proxyport).
Acquire::http::Proxy “http://username:[email protected]:proxyport”;
Save the apt.conf file.

References :
http://askubuntu.com/questions/257290/configure-proxy-for-apt
http://askubuntu.com/questions/543616/why-does-add-apt-repository-now-fail-to-retrieve-keys-behind-my-proxy-server-bu

Any Comments?