[tex]a)\frac{2}{n+1}\geq\frac{1}{2}\\
4\geq n+1\\
3\geq n\\
n\in N\Rightarrow S:n\in \{0,1,2,3\}\\
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b)\frac{3n+1}{2}\leq \frac{21}{6}\\
18n+6\leq 42\\
18n\leq 36\\
n\leq 2\\
n\in N\Rightarrow S:n\in \{0,1,2\}\\
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c)\frac{5}{3}\leq \frac{21}{2n+1}\\
10n+5\leq 63\\
10n\leq 58\\
n\leq5,8\\
n\in N\RightarrowS:n\in \{0,1,2,3,4,5\}\\
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d)\frac{6n+1}{3}\leq \frac{8n+3}{4}\\
24n+4\leq24n+9\\
4\leq9(A)\Rightarrow S:n\in N[/tex]