Răspuns:
[tex]\frac{6x+7}{7x+2}[/tex] se simplifica <=> 7x+2 | 6x+7
7x+2 | 7x+2
<=> 7x+2 | 7*(6x+7)
7x+2 | 6*(7x+2) 7,6 ∈ N
<=> 7x+2| 42x+49
7x+2| 42x+12 ===> 7x+2 | (42x+49)-(42x+12)
=> 7x+2 | (42x+49-42x-12)
=> 7x+2 | 37 => 7x | 35 => x|5 si x∈N => x∈ D₅= {1,5}
ptr x=1, obtinem 13/9 care nu se simplifica
ptr x=5, obtinem 37/37=1 - se simplifica
=> x=5 ∈N