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There a few download/decompression programs for the iPad that will (reportedly) handle .rar files. A search in the App Store for '.rar' turned up some. I've not experience with any of them, so I suggest you perform the search yourself and see which one looks to work best for you.
Hold in mind that whatever is in the .rar file will only be useable on the iPad if you have an app for that file type as well, and it registers itself with the system to say so. The decompressing app you use will also need to support Open In to move said apps to the compatible app. Most should have this.