$\text{Bài 1:}$
$\text{A = {15; 20; 25; 30; 35; 40; 45; ... ; 90; 95}}$
$\text{Bài 2:}$
$\text{a) A = {x ∈ N/1 < x < 10}}$
$\text{b) B = {x ∈ N/44 < x < 52}}$
$\text{c) C = {x ∈ N/ x = 2k; 0 < x < 22}}$
$\text{d) D = {x ∈ N/ x = 3k; 0 ≤ x < 30}}$
$\text{e) E = {x ∈ N/ x = 3k + 1; 0 < x < 110}}$