i lose my pages app after i restored my macbook air

Depending on where you got it and what OS your computer came with, install it from an iWork disk, your computer's original thumbdrive, or the Mac App Store. If you got it with a computer that shipped with Mac OS X 10.7 or newer but didn't associate it with the Mac App Store, click here.

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Open the App Store > Purchased

Should be there if you bought the app or if it came with your computer. Make sure you're signed into whichever Apple ID you purchased/accepted Pages with.

If not, do what Niel said. 🙂