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The United States of America is a federal republic [ 10 ] [ 11 ] consisting of fifty states and a federal district , as well as several territories . [ 12 ] It is commonly called the United States ( US , USA , U.S. or U.S.A. ) and America . The country is situated mostly in central North America , where its forty-eight contiguous states and Washington, D.C. , the capital district , lie between the Pacific and Atlantic Oceans , bordered by Canada to the north and Mexico to the south. The state of Alaska is situated in the northwest portion of the continent, while the state of Hawaii is an archipelago in the Mid Pacific.
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Nanjing University of Aeronautics and Astronautics