Just bought ann app,when i touch the icon,it just goes full screen,then disappears why.

Sometimes, the iPad doesn't react well when you install an app (and no, can't tell you why or when it'll happen).

Try doing a re-boot of the iPad (hold the sleep and home buttons until the iPad screen goes black, keep holding until the white apple icon comes back on, then let both buttons go). It'll take a bit to finish re-booting...

Re-booting usually takes care of a lot of problems. Let us know if that solves your app problem.

Good luck.

Marilyn