Question

A= { x | x € N si (x+1) | (x+7) }

Answer 1

x + 1 | x +7 
x + 1 | ( x + 1) + 6 
daca  x +1 | 6 
x +1 =1                 x =0 
x  + 1=2                 x =1
x +1 =3                  x = 2 
x +1 = 6                 x = 5 
A = { 0 ; 1 ; 2 ; 5} 

Answer 2

X+7-x-1=6. X apartinr D 6(1,2,3,6). X+1=1=>x=0; x+1=2=>x=1; x+1=3=>x=2; x+1=6=>x=5. A={0,1,2,5}