How can I close the application when I used it ?
Double click home button, swipe previews upwards, not small icons.
Double tap the Home button and then upward swipe the App Preview and it will fly closed.
Note that an application, in almost all cases, is not still "running" when you switch to another application.
but with ios 7, it doesn't close the application and the application still "running". exemple skype...
Check specifically for Skype in your Settings and see if it is somehow set to always be 'open' to accept incoming calls.
okay, thank's. I will try.
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