$\text{a, Có: a+4b$\vdots$13}$
$\text{→10(a+4b)$\vdots$13}$
$\text{Có: 10(a+b)=10a+40b=10a+b+39}$
$\text{Vì tổng trên có 39b$\vdots$13→10a+b$\vdots$13(ĐPCM)}$
$\text{b,Có: 2(10a+b)-(3a+2b)=20a+2b-3a-2b=17a}$
$\text{Vì 17$\vdots$17→17a$\vdots$17}$
$\text{→2(10a+b)-(3a+2b)$\vdots$17}$
$\text{Vì 3a+2b$\vdots$17→2(10a+b)$\vdots$17}$
$\text{Mà (2,17)=1→10a+b$\vdots$17}$
$\text{→Nếu 3a+2b$\vdots$17 thì 10a+b$\vdots$17}$
#Zie