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Math Help - showing this is a multiple of 9

  1. #1
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    showing this is a multiple of 9

    show (n-1)^3+n^3+(n+1)^3 is a multiple of 9

    I expanded and have got 3n^3+6n so I need to show this is divisible by 9

    3n^3+6n=9((1/3)n^3+(2/3)n)) but I can not gurantee the thing in the bracket is an integer
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  2. #2
    Super Member TheChaz's Avatar
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    Quote Originally Posted by poirot View Post
    show (n-1)^3+n^3+(n+1)^3 is a multiple of 9

    I expanded and have got 3n^3+6n so I need to show this is divisible by 9

    3n^3+6n=9((1/3)n^3+(2/3)n)) but I can not gurantee the thing in the bracket is an integer
    3n^3 + 6n = 3n(n^2 + 2)... I just factored this for ease in computation.
    Now consider the remainder when n is divided by three.
    n = 3k
    n = 3k + 1 or
    n = 3k + 2.
    If n = 3k, then our expression 3n(n^2 + 2) = 3*(3k)*(9k^2 + 2) = 9k*something, is divisible by 9 (obviously).
    If n = 3k + 1, then our expression 3n(n^2 + 2) = 3(3k + 1)*[(3k + 1)^2 + 2] = 3(3k + 1)*[3(3k^2 + 2k + 1)] = 9*something
    If n = 3k + 2 is similar
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  3. #3
    Forum Admin topsquark's Avatar
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    Quote Originally Posted by poirot View Post
    show (n-1)^3+n^3+(n+1)^3 is a multiple of 9

    I expanded and have got 3n^3+6n so I need to show this is divisible by 9

    3n^3+6n=9((1/3)n^3+(2/3)n)) but I can not gurantee the thing in the bracket is an integer
    You have two good solutions already so I will just mention that induction is also a fairly easy way to do this as well.

    -Dan
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