Em tham khảo
$\text{a) Xét hiệu:{5n-1}-5{n+2} ÷d}$
$⇒5n-1-5n-2 ÷d$
$⇔-3÷d$
$⇔d∈Ư(-3)={±3}$
$\text{Vậy d={±3}}$
$\text{b) Vì x÷3 dư 2}$
$⇒x-2÷3$
$⇒x-2∈B(3)={0,3,6,9,12,15,18,...}$
$\text{Vì 0<x<20}$
$⇒x-2={3,6,9,12,15,18}$
$⇒x={5,8,11,14,17,20}$
$\text{Vì 0<x<20}$
$\text{Vậy x={5,8,11,14,17}}$