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Math Help - Arithmetic proof

  1. #1
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    Arithmetic proof

    Prove that, for all n, n^3 + 2n^2 - n - 1 is not divisible by 3.

    I hate to ask a question like this...but how do I get started here? I don't even know where to start. Could anyone just tell me the first starting steps in order to prove this and/or the concept that would prove this true? Any help would be appreciated.
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  2. #2
    MHF Contributor Drexel28's Avatar
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    Quote Originally Posted by SterlingM View Post
    Prove that, for all n, n^3 + 2n^2 - n - 1 is not divisible by 3.

    I hate to ask a question like this...but how do I get started here? I don't even know where to start. Could anyone just tell me the first starting steps in order to prove this and/or the concept that would prove this true? Any help would be appreciated.
    You need only check it for the cases n=0,1,2 where you get -1,1,13 respectively.
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