a) 4 : (x - 1);
=> x - 1  ∈ U(4)
=> x - 1 ∈ {-4 ; -2; -1; 1; 2; 4}
=> x ∈ {-3; -1; 0 ; 2; 3; 5}
b) 15 : (2x + 1);
=> 2x + 1  ∈ Ư(15)
=> 2x + 1 ∈ {-15 ; -5; -3; -1; 1; 3; 5; 15}
=> 2x ∈ {-16; -6; -2 ; 0; 2; 4; 14}
=> x ∈ {-8; -3; -1; 0 ;1 ;2;7}
c) (x + 17) : (x + 2).
=> (x + 2 + 15) : (x + 2).
=> x + 2  ∈ Ư(15)
=> x + 2 ∈ {-15 ; -5; -3; -1; 1; 3; 5; 15}
=> x ∈ {-17; -7; -5; -3; -1; 1; 3; 13}

a) x-1 thuộc Ư(4) = {1,2,4,-1,-2,-4}
x  = 2, 3, 5, 0, -1, -3
b) 2x + 1 thuộc Ư(15) = {1,3,5,15,-1,-3,-5,-15}
2x = 0, 2, 4,14, -2, -4, -6,-16
x = 0,1,2,7,-2,-3,-8
c) (x + 17 ) : ( x + 2 )
(x + 2 + 15 ) : ( x+ 2 ) 
Vì x + 2 chia hết cho x + 2 nên 15 chia hết cho x + 2 
Suy ra x + 2 thuộc Ư(15)= { 1,3,5,15,-1,-3,-5,-15}
          x = -1,1,3,13,-3,-5,-17