This is the third reinstall of Ubuntu 14.04 I have done today.
I just bought a laptop with an Core i3 processor and an Nvidia GeForce GT 840M graphics card.
I have installed the Nvidia driver using their .run file, and also following tutorials, and it just doesn't work. When I check in the terminal which GPU I am using it says "i915" instead of "nvidia".
Uninstall the NVIDIA proprietary graphics driver. Find the file you used to install the NVIDIA graphics driver from the NVIDIA website. It's in whatever folder you downloaded it to (your Downloads folder, for example). It should be named something like NVIDIA-Linux-x86_64-340.58.run. If so, the command to uninstall it is:
sudo ~/Downloads/NVIDIA-Linux-x86_64-340.58.run --uninstall
sudo reboot
If the NVIDIA-Linux-x86-XXX.XX.run file has a different name or location, then change the name and/or location in the above command. The name and location of the .run file in the above command have to match exactly to the name and location of the .run file that is in your computer.
Run the following command to identify the recommended NVIDIA proprietary graphics driver:
ubuntu-drivers devices
Install the recommended NVIDIA proprietary graphics driver and reboot.
NVIDIA Prime is a way of adding hybrid graphics support to your laptop. NVIDIA Prime allows the user to switch between NVIDIA (Performance Mode) and Intel (Power Saving Mode) graphics from the NVIDIA Settings utility.
If your computer has an NVIDIA 319 or more recent graphics driver installed, run the following command to install NVIDIA Prime packages:
sudo apt-get install nvidia-settings nvidia-prime
Then NVIDIA Prime can be enabled/disabled from the NVIDIA X Server Settings application.
I found a solution in this post: https://askubuntu.com/a/557395/321613
This works well for me, as I have an Axus UX303LN with Nvidia GT840M graphic card.
