Question

fie multimile a={x€n | 12 suprax+1€ n}

Answer 1

A={x∈ N I  12/x+1∈N}
12/x+1∈N⇒x+1∈D12
D12∈{1,2,3,4,6,12}

1. x+1=1
    x=1-1
    x=0

2. x+1=2
    x=2-1
    x=1

3. x+1=3
    x=3-1
    x=2

4.  x+1=4
     x=4-1
     x=3

5.  x+1=6
     x=6-1
     x=5

6.   x+1=12
      x=12-1
      x=11

A={0,1,2,3,5,11}