Đáp án:
$\begin{array}{l}
a)a + 1 = a - 1 + 2\\
Do:\left( {a - 1} \right) \vdots \left( {a - 1} \right)\\
\Rightarrow 2 \vdots \left( {a - 1} \right)\\
\Rightarrow \left( {a - 1} \right) \in Ư\left( 2 \right) = \{ - 2; - 1;1;2\} \\
\Rightarrow a \in \{ - 1;0;2;3\} \\
b)\left( {5x + 13} \right) \vdots \left( {2x + 1} \right)\\
\Rightarrow 2\left( {5x + 13} \right) \vdots \left( {2x + 1} \right)\\
Do:2\left( {5x + 13} \right) = 10x + 26 = 10x + 5 + 21\\
= 5\left( {2x + 1} \right) + 21\\
Do:5\left( {2x + 1} \right) \vdots \left( {2x + 1} \right)\\
\Rightarrow 21 \vdots \left( {2x + 1} \right)\\
\Rightarrow \left( {2x + 1} \right) \in Ư\left( {21} \right) = \{ - 21; - 7; - 3; - 1;1;3;7;21\} \\
\Rightarrow x \in \{ - 11; - 4; - 2; - 1;0;1;3;10\}
\end{array}$
