Cách giải:
$a,x-12 \vdots x-5$
$\to x-5-7 \vdots x-5$
$\to 7 \vdots x-5$
$\to x-5 \in Ư(7)={1,-1,7,-7}$
$\to x \in {6,4,12,-2}$
$b,2x+7 \vdots x+3$
$\to 2(x+3)+1 \vdots x+3$
$\to 1 \vdots x+3$
$\to x+3 \in Ư(1)={1,-1}$
$\to x \in {-2,-4}$
$c,4x+21 \vdots x+1$
$\to 4(x+1)+17 \vdots x+1$
$\to 17 \vdots x+1$
$\to x+1 \in Ư(17)={1,-1,17,-17}$
$\to x \in {0,16,-18,-2}$
$36 \vdots 2x-1$
$\to 2x-1 \in Ư(36)={1,-1 2,-2,3,-3,4,-4,6,-6,9,-9,12,-12,18,-18,36,-36}$
Mà 2x-1 là số lẻ
$\to 2x-1 \in {1,-1,3,-3,9,-9}$
$\to 2x \in {0,2,4,-2,10,-8}$
$\to x \in {0,1,2,-1,5,-4}$
