Cho: $a \vdots 3+b$ $5-a+b \vdots a+13$ Vậy $a.2b$ chia hết cho ………

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1. Vì $a\vdots 3+b$
   $\Rightarrow 3+b\in Ư(a)$
   $\Rightarrow 3+b=a\vdots a$
   Thay $a=3+b$ vào $5-a+b$, ta được:
   $5-a+b \vdots a+13$
   $\Leftrightarrow 5-(3+b)+b\vdots a+13$
   $\Leftrightarrow 5-3-b+b\vdots a+13$
   $\Leftrightarrow 2\vdots a+13$
   $\Rightarrow (a+13)\in Ư(2)=\left \{ ±1;±2 \right \}$
   $\Rightarrow a\in \left \{ -11;-12;-14;-15 \right \}$
   $\Rightarrow (3+b)\in \left\{\begin{matrix}
   Ư(-11)=\left \{ ±1;±11 \right \}\\ 
   Ư(-12)=\left \{ ±1;±2;±3;±4;±6;±12 \right \}\\ 
   Ư(-14)=\left \{ ±1;±2;±7;±14 \right \}\\ 
   Ư(-15)=\left \{ ±1;±3;±5;±15 \right \}
   \end{matrix}\right.$
   $\Rightarrow b\in \left \{ -1;-4;8;-14;-5;0;-6;1;-7;3;-9;9;-15;4;;11;-17;2;-8;12;-18 \right \}$
   $\Rightarrow a.2b\vdots …$

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