Giải thích các bước giải:
a.Ta có:
$x+13\quad\vdots\quad x+1$
$\to (x+1)+12\quad\vdots\quad x+1$
$\to 12\quad\vdots\quad x+1$
$\to x+1\in U(12)$
Vì $x\in N^*\to x\ge 1\to x+1\ge 2$
$\to x+1\in\{2,3,4,6,12\}$
$\to x\in\{1,2,3,5,11\}$
b.Ta có:
$2x+100\quad\vdots\quad x+1$
$\to (2x+2)+98\quad\vdots\quad x+1$
$\to 2(x+1)+98\quad\vdots\quad x+1$
$\to 98\quad\vdots\quad x+1$
$\to x+1\in U(98)$
Mà $x\in N^*\to x\ge 1\to x+1\ge 2$
$\to x+1\in\{2,7,14,49,98\}$
$\to x\in\{1,6,13,48,97\}$
