Step 1 : enter xrandr in your terminal. 

You will see that there are several outputs which are connected. The LVDS1 is your laptop’s screen. Normally, there is another output called VGA-1 orVGA-2 which is also connected. You have to note the name of this output.

Step 2 : Edit grub with gksudo gedit /etc/default/grub GRUB_CMDLINE_LINUX="video=VGA-2:d

If your output is called like VGA-1-1, set video=VGA-1:d (setting video=VGA-1-1:d won’t work).

Step 3 : sudo update-grub

Step 4 :reboot

source : askubuntu