A={n∈N/ n/n+9}
n/n+9
n/n (-)
=> n/n+9-n, n/9 => n∈{1, 3, 9}
A={1, 3, 9}
B={n∈N/ n+1/n+6}
n+1/n+6
n+1/n+1 (-)
=> n+1/n+6-(n+1)
n+1/5 => n+1∈{1, 5}=> n∈{0, 4}
B={0, 4}
C={n∈N/ n-3/n}
n-3/n
n-3/n-3 (-)
=> n-3/n-(n-3)
n-3/3 => n-3∈{1, 3}; n∈{4, 6}
C={4, 6}
D={n∈N/ 2n+1/5n+8}
2n+1/5n+8 /*2
2n+1/2n+1 /*5
=> 2n+1/10n+16
2n+1/10n+5 (-)
=> 2n+1/11 => 2n+1∈{1, 11}
2n∈{0, 10} => n∈{0,5}
D={0, 5}