How do I activate my warranty on ipad

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Warranty is automatically activated when you purchase your iPad Air, so do not worry about it. Also, during the setup your iPad Air is going to be registered to your Apple ID.

You do not need to put the serial number anywhere. However, if you got AppleCare from a reseller or a third-party shop alongside your iPad, you need to register it > https://selfsolve.apple.com/Agreements.do If you got AppleCare at an Apple Store alongside your iPad, it was probably activated

If you've set your device up and actiavated it then you've activated the warranty. The warranty starts from the purchase date.

beyond that you can find a lot of answers here

http://www.apple.com/support/ipad/

As was mentioned, your device should be registered when activated under your Apple ID. You can check and verify if your device is registered at the website below. You'll need to login to your Apple ID so have your credentials ready. From there, you can also register a device that has not yet been registered:

https://supportprofile.apple.com/