Đáp án:
 
Giải thích các bước giải:
 
 
$\bullet \,\,\,\,a-b\,\,\,\vdots \,\,\,7$
$\to 4\left( a-b \right)\,\,\,\vdots \,\,\,7$
$\to 4a-4b\,\,\,\vdots \,\,\,7$
$\to 4a-4b+7b\,\,\,\vdots \,\,\,7$
$\to 4a-3b\,\,\,\vdots \,\,\,7$
 
 
$\bullet \,\,\,\,m+4n\,\,\,\vdots \,\,\,13$
$\to 10\left( m+4n \right)\,\,\,\vdots \,\,\,13$
$\to 10m+40n\,\,\,\vdots \,\,\,13$
$\to 10m+40n-39n\,\,\,\vdots \,\,\,13$
$\to 10m+n\,\,\,\vdots \,\,\,13$