Proof of Parallel Axis Theorem

Note: The following are taken from Physics for Scientists and Engineers 6E

http://img542.imageshack.us/img542/821/75796098.png [Broken]
http://img152.imageshack.us/img152/5615/70724407.png [Broken]
http://img804.imageshack.us/img804/6813/65815357.png [Broken]

I don't really understand why is that integral equals to 0.

Can anyone explain? Thanks!

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