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Useful to copy: This is not research-level math; further, it looks like homework. As such, it does not belong on mathoverflow, which is for research-level problems. It may be better suited for math.stackexchange; however, if you do choose to post there, you should give more context - especially by showing what you have tried to do and where you are stuck. See math.stackexchange.com/help/how-to-ask for more information.