plus f) conform fractiei 13/(x+2) se impune ca x+2 apartine D13
D13 ={-13,-1, 1, 13}
x+2= -13 => x= -11 nu apartine N
x+ 2= -1 => x= -3 nu apartine N
x+2= 1=> x= -1 apartine N
x+2= 13 => x= 11 apartine N
deci F={-1,11}
