some short and selected math problems of different levels in random order I try to keep the ans simple
we have
$2n^3+3n^2 + n = n(2n^2+3n +1) = n(2n+1)(n+1)$
$= n((n-1) + (n+2))(n+1) = (n-1)n(n+1) + n(n+1)(n+2)$
$(n-1)n(n+1)$ is product of 3 consecutive integers and hence divisible by 6 and so is $n(n+1)(n+2)$ and hence the sum is divisible by 6
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