since changing the http_proxy, https_proxy and ftp_proxy environment variables didn’t work out with me, I had to search and find another way, and it worked flawlessly:

1- Add a new file with name “80proxy” to “/etc/apt/apt.conf.d”, e.g. “vi /etc/apt/apt.conf.d/80proxy”
2- Add the following lines:

Acquire::http::proxy “http://<proxy&gt;:<port>/”;
Acquire::ftp::proxy “ftp://<proxy>:<port>/”;
Acquire::https::proxy “https://<proxy&gt;:<port>/”;

3- That’s it, now I’m a happy apt-get user :))