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Kaya Renger Coordinator

11/08/2017 at 14:54
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Prove that :) with n \(\in\) Z , so that :

n2.(n4 - 1) \(⋮\) 60 




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    Phan Thanh Tinh Coordinator 11/08/2017 at 15:38

    Denote A = n2(n4 - 1) = n2(n2 - 1)(n2 + 1) is the product of 3 consecutive integers,so \(A⋮3\).We have :

    \(\circledast A=n^2\left(n-1\right)\left(n+1\right)\left(n^2+1\right)\)

    If n is even,then \(n^2⋮4\) and \(A⋮4\)

    If n is odd,then n - 1 and n + 1 is even. So,\(A⋮4\)

    Hence,\(A⋮4\)

    \(\circledast A=n^2\left(n^2-1\right)\left(n^2-4\right)+5n^2\left(n^2-1\right)\)

    \(=\left(n-2\right)\left(n-1\right)n^2\left(n+1\right)\left(n+2\right)+5n^2\left(n^2-1\right)\)

    \(\left(n-2\right)\left(n-1\right)n^2\left(n+1\right)\left(n+2\right)\)include the product of 5 consecutive integers,so it's divisible by 5.Moreover, \(5n^2\left(n^2-1\right)⋮5\)

    Hence,\(A⋮5\)

    Since A is divisible by 3,4,5 and 3,4,5 are relatively prime numbers, \(A⋮3.4.5=60\)

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