3n+40 chia hết cho n + 4

`3n+40vdotsn+4`

`=>3n+12+28vdotsn+4`

`=>3(n+4)+28vdotsn+4`

Vì `3(n+4)vdotsn+4`

`=>28vdotsn+4`

`=>n+4 in Ư(28)={+-1;+-2;+-4;+-7;+-14;+-28}`

Ta có bảng sau:

\begin{array}{|c|c|c|c|c|c|c|c|c|c|c|c|c|}\hline \text{$n+4$}&\text{1}&\text{$-1$}&\text{2}&\text{$-2$}&\text{4}&\text{$-4$}&\text{7}&\text{$-7$}&\text{14}&\text{$-14$}&\text{28}&\text{$-28$}\\\hline \text{$n$}&\text{$-3$}&\text{$-5$}&\text{$-2$}&\text{$-6$}&\text{0}&\text{$-8$}&\text{3}&\text{$-11$}&\text{10}&\text{$-18$}&\text{24}&\text{$-32$}\\\hline\end{array}

Vậy `n in {-32;-18;-11;-8;-6;-5;-3;-2;0;3;10;24}`