How to run command apt-get update behind proxy

Before we setting the proxy on ubuntu

root@ubuntu:/tmp#
root@ubuntu:/tmp# sudo apt-get update
Ign:1 http://security.ubuntu.com/ubuntu xenial-security InRelease
Ign:2 http://security.ubuntu.com/ubuntu xenial-security Release
Ign:3 http://security.ubuntu.com/ubuntu xenial-security/main amd64 Packages
Ign:4 http://security.ubuntu.com/ubuntu xenial-security/main i386 Packages
Ign:5 http://security.ubuntu.com/ubuntu xenial-security/main all Packages
..
..
E: Failed to fetch http://id.archive.ubuntu.com/ubuntu/dists/xenial-updates/main/binary-amd64/Packages  Unable to connect to id.archive.ubuntu.com:http: [IP: 91.189.88.149 80]
E: Failed to fetch http://id.archive.ubuntu.com/ubuntu/dists/xenial-backports/main/binary-amd64/Packages  Unable to connect to id.archive.ubuntu.com:http: [IP: 91.189.88.149 80]
E: Some index files failed to download. They have been ignored, or old ones used instead.
sysop@ubuntu:/tmp$
sysop@ubuntu:/tmp$ 

we create apt.conf file and add the following config as shown bellow

sysop@ubuntu:/etc/apt/apt.conf.d$ cd ..
sysop@ubuntu:/etc/apt$ sudo vi /etc/apt/apt.conf
sysop@ubuntu:/etc/apt$