<s>[INST] Find the number of integers between 1 and 25 so that the expression is divided by 6 Find the number of integers so that the $n^2+3n+2$ is divided by 6 when $1 \le n \le 25$.
I start by $6\mid n^2+3n+2 6|(n+1)(n+2) \implies  6k=(n+1)(n+2) $ when $ k \in Z$
we can see that for $n=$ $1,2,4,5,7,8,10,11....$ will work, but I am not sure how to continue from here .
(some HINT can be very appreciated ).
Additionally I would love see a combinatorics approach for this .
Thank you guys.
 [/INST] Being divisible by $6$ means being even and divisible by $3$.


*

*Modulo $2$, $n^2+3n+2\equiv n^2+n\equiv n+n \text{ (by Fermat) }\equiv 0$. So it is always even.

*Modulo $3$, $n^2+3n+2\equiv n^2+2$, and it is congruent to $0$ if and only if $n^2\equiv 1\mod 3$, which happens only if $n\equiv \pm 1$, in other words, if and only if $n$ is not divisible by $3$.


So we simply can take the list of integers between $1$ and $25$ and remove the $8$ multiples of $3$ from this list. There remains $17$ integers.
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