The ultimate app to pay for is here. Duet Display! Use your ipad as an extra screen to your laptop.
What do you need? A mac with 10.9 or later or a PC with Win7 or later.
You need to buy the app on your iPad/appstore. No its not free but cheap, at the moment 107 SEK and belive me ITS WORTH IT!!
Ok install the app on your iPad. On your laptop go to http://www.duetdisplay.com/ and download the other part (free). Install! Restart.
Use you USB synccable that came with the Ipad and BANG, it works. You can tell that the developers are former Apple employes because its so easy to install and use and on their webpage,
clear and easy to understand. I declaire this best app 2015!!!