Question

Det. nr. nat. X, astfel incat 18 supra 2X+1 apartine N

Answer 1

$\frac{18}{2x+1}$ ∈ N ⇔
2x+1 | 18 ⇒
2x+1 ∈ {1,2,3,6,9,18}
2x ∈ {0,1,2,5,8,17}
x ∈ {0, 1/2, 1, 5/2, 4, 17/2}
Dar x este numar natural deci 
x ∈ {0,1,4}

Answer 2

18/2x+1∈N;
⇒2x+1∈d18=[1,2,3,6,9,18];
⇒2x-numar par;
⇒1-numar impar;
⇒numar par+numar impar=numar impar;
⇒2x+1=[1,3,9]
⇒2x=0⇒x=0;
⇒2x=2⇒x=1;
⇒2x=8⇒x=4;
⇒x=[0,1,4];