Useful links:https://www.qgis.org/en/site/forusers/alldownloads.htmlhttp://askubuntu.com/questions/351899/qgis-quantum-gis-install-fails-unmet-dependencieshttp://blog.csdn.net/heybob/article/details/529113951. First remove already added repositories which you tried to install Qgis:

Execute this in your terminal: sudo software-properties-gtk and go to Other Softwaretab.
Then remove those repositories that you added using your mentioned tutorial. Find names similar to http://qgis.org/debian and ubuntugis-unstable. Close the window.
The execute this command in terminal to update your repository: sudo apt-get update

2. Now add the QGIS repository once again to your /etc/apt/sources.list file. Do it by following these steps:

Open terminal and execute: sudo gedit /etc/apt/sources.list

Add following lines at the last line: deb http://qgis.org/ubuntugis trusty main deb http://ppa.launchpad.net/ubuntugis/ubuntugis-unstable/ubuntu trusty main
Save the file and close the window.
Execute following two commands to add public key of QGIS wget -O - http://qgis.org/downloads/qgis-2016.gpg.key | gpg --import gpg --fingerprint 073D307A618E5811 After you have verified the fingerprint you can add the key to apt with: gpg --export --armor 073D307A618E5811 | sudo apt-key add -
Update repository by executing following command: sudo apt-get update

3. Now install QGIS by following these steps:

First remove already installed QGIS or any residual( and broken) packages if any: sudo apt-get autoremove sudo apt-get autoclean sudo apt-get -f install sudo apt-get autoremove qgis sudo apt-get --purge remove qgis
Install grass plugin: sudo apt-get install grass sudo apt-get install qgis-plugin-grass
Finally install QGIS: sudo apt-get install qgis python-qgis

when run qgis, there are some warnings; it shows no module named PyQt4.qgis works fine now. I will update if the warning cause some problems in the future.