Explicație pas cu pas:
a)
[tex]\frac{1}{3} < \frac{n}{12} < \frac{3}{4} \iff \frac{4}{12} < \frac{n}{12} < \frac{9}{12} \\ \iff 4 < n < 9 \implies \bf n \in \{5 ; 6 ; 7 ; 8\}[/tex]
b)
[tex]\frac{1}{9} < \frac{1}{2n + 1} < 1 \iff \frac{1}{9} < \frac{1}{2n + 1} < \frac{1}{1} \\ \iff 9 > 2n + 1 > 1 \iff 8 > 2n > 0 \\ \implies 4 > n > 0 \implies \bf n \in \{1 ; 2 ; 3\}[/tex]
