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When ever you exit an app using home key, the app gets saved in its current state and remains active in the back ground. This means that app is utilising the phones resources thus costing phones battery life.
The best way to exit app is by pressing the back key, the app will close in its current state (although it would still be available in the recent apps list ), but if you think that your phone is running slow and you need to make it faster by closing unwanted apps, you need to press and hold the home button this will open the task manager. Then you can just slide an app left or right using your finger to throw it off the screen. This will wipe off any trace of the app from the RAM and will free up resources for other tasks to run (this will not delete the app). Freeing up resources will make your phone run faster.
You can access the recently used app by pressing and holding the home key.
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